www.elsevier.com/locate/scitotenv

Science of the Total Environm

Effect of policy-induced measures on suspended sediments

and total phosphorus concentrations from three

Norwegian agricultural catchments

Marianne Bechmanna,T, Per Stalnackeb

aJordforsk-Norwegian Centre for Soil and Environmental Research, FA. Dahls vei 20, 1432 As, NorwaybNIVA-Norwegian Institute for Water Research, PO Box. 173, 0411 Oslo, Norway

Available online 17 March 2005

Abstract

In Norway, agricultural subsidies have, since the late 1980s, been targeted to reduce soil erosion, transfer of soil particles and

phosphorus (P) losses. The subsidies led to, e.g., a fourfold increase in the area not ploughed from 1991 to 2001 and a reduced P

fertiliser consumption by 60%, especially in areas with high livestock density. Moreover, in the late 1980s agricultural point

sources of P from storage facilities of manure and fodder were reduced. In this paper, we evaluate the effect of these policy-

induced measures and changed agricultural practices on suspended sediment (SS) and total P (TP) concentrations in three

agricultural catchments (1, 3 and 87 km2). Results from the statistical trend analyses for the study period (14–17 years) showed

weak, but statistically significant ( pb0.05), downward trends in concentrations of TP and SS in the two streams with a high

initial TP or SS concentration. In the stream with low initial concentrations of TP and SS, however, no statistically significant

trends were shown. The stream with the highest initial concentration of SS showed a statistically significant downward trend in

both TP and SS concentrations. The catchment with low initial concentration of SS and medium livestock density showed no

detectable trends, while the catchment with high livestock density and low concentrations of SS in the stream showed a

statistically significant downward trend in TP concentrations. The results from this study suggest that subsidies and mitigation

measures can reduce concentrations of TP and SS in streamwater in highly polluted catchments, although the reduction is small

compared to the variations between catchments.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Phosphorus; Suspended sediments; Autumn tillage; Livestock density; Subsidies; Catchments; Norway

0048-9697/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.scitotenv.2005.02.013

T Corresponding author. Tel.: +47 64948100; fax: +47 64948110.

E-mail address: marianne.bechmann@jordforsk.no

(M. Bechmann).

1. Introduction

In Norway, mitigation of soil erosion, transfer of

soil particles and phosphorus (P) transfer have been in

focus since the early 1980s because of the adverse

effects on water quality in inland surface waters. In

ent 344 (2005) 129–142

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142130

large parts of the agricultural areas in Norway, soil

erosion is the main process of P losses from arable

land. Because of the agricultural policy, these areas

are mainly used for cereal production with a relatively

low livestock density. In other areas, dairy production

was stimulated during the 1960–1970s, resulting in

higher P surplus in agricultural production (Haraldsen

et al., 1995). High P application rate, resulting in

increased risk of P losses, was here a main challenge

to the environment. In 1985, the national authorities

introduced the National Action Plan against Agricul-

tural Pollution (1985–1988), which in the late 1980s

resulted in practical measures and increased political

interest (Lundekvam et al., 2003). Priority was first

given to the establishment and improvement of

municipal wastewater treatment facilities to mitigate

point sources of P. To mitigate sources of diffuse P,

during the late 1980s and 1990s, several economic

incentives (e.g., subsidies and direct payment) were

introduced to stimulate the farmers to implement

measures to reduce soil and P losses. Measures were

separated into addressing P source and P transport

factors, and included measures like reduced autumn

tillage, nutrient management planning and increased

area of vegetated buffer zones and grassed waterways

(Lundekvam et al., 2003) (Table 1). Agricultural

legislation was also changed, i.e., (i) the maximum

livestock density on each farm was set to 2.5 livestock

units (LU) ha�1 and, (ii) the time and method of

manure application were regulated, e.g., on arable

land manure must be incorporated within 18 h after

application; as well as seasonal regulations (Table 1).

As a result, the proportion of manure spread during

Table 1

Agricultural policy measures for mitigating phosphorus (P) losses

from agriculture in Norway (Norwegian Ministry of Agriculture,

2003)

Source measures Transport measures

Livestock density;

2.5 AMU ha�1

Reduced autumn tillage

Capacity for manure

storage for 8 months

Contour ploughing

No manure application November

1st to February 15th

Vegetated buffer strips

Incorporation of manure

within 18 h on arable land

Hydrological measures

(e.g., inlet for surface water

to the drainage system)

Nutrient management plan Grassed waterways

spring and the growing season increased in Norway

during the 1990s, and constituted 80% of the total

amount of manure spread on agricultural fields in

2002 (Bye et al., 2003). In addition, the new

regulations also required the farmers to establish an

annual nutrient management plan, based on analyses

of soil P status every 5th to 8th year. Moreover, in the

late 1980s, authorities inspected the farms in order to

identify sources of loss from manure and fodder

storage facilities.

In order to gauge the effectiveness of policy

changes, it is essential to determine if and how

successful these environmental measures are, and

how long it may take to detect the response in

agricultural streams. The effects of various measures

on water quality are mainly reported by results

obtained from plot and field experiments (Eltun et

al., 1996; Sharpley and Rekolainen, 1997; Øygarden,

2000). Studies on the effect of measures are however

often limited in time and space. This normally limits

general conclusions and extrapolations beyond the

considered study period and area. Investigation of

long-term time series at the catchment scale can be

used to evaluate mitigation strategies. In the wider

Nordic–Baltic region, the effect of agri-environmental

measures has been studied by the use of long-term

monitoring data in agriculturally impacted rivers and

streams. The results from these studies are not

conclusive. For example, Mander et al. (2000)

showed decreased P loads in some Estonian streams

and related these to the dramatic changes in land

management, i.e., an increased area of abandoned

land, while another similar study of 22 Estonian

rivers and streams only detected significant down-

ward trends in P concentration in 2 sites and even

reported increased concentrations at 2 sites (Iital et

al., 2005). Significant downward trends in P concen-

trations have been reported in Latvian rivers (Stal-

nacke et al., 2003) and small agricultural catchments

in Denmark (Larsen et al., 1999). These decreases

were, however, attributed to a reduction in point

sources from municipal wastewater treatment plants

(Latvia) and a reduction in loss of P from scattered

dwellings (Denmark). The contribution of P from

agriculture was evaluated in Finnish rivers over a 25-

year period and Raike et al. (2003) found no clear

effects of decreasing non-point source loading. In

Sweden, field scale monitoring showed no obvious

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142 131

trends in P losses as a result of negative P balance in

agricultural production during the 1970s and 1980s

(Ulen et al., 2001). Historically high P surplus on

these fields may explain the missing trends. During

the 1990s, trends in losses of suspended sediment (SS)

and total phosphorus (TP) were difficult to detect in

Norwegian streams dominated by agricultural contri-

butions, since the most efficient measures were

implemented before this period (Stalnacke and Bech-

mann, 2002). To summarise, only few studies describe

changes in agricultural management within the catch-

ments, which could account for significantly decreas-

ing trends in SS and TP.

The aim of this study is to evaluate the effect of

measures implemented to reduce losses of SS and TP

from agricultural areas. We analysed three long-term

time series (14–17 years) of data on SS and TP

concentrations in agricultural streams representing

areas with i) cereal production, high erosion risk and

low livestock density, ii) cereal production, low

erosion risk and medium livestock density and iii)

Fig. 1. Map of Norway and the Rbmua (8710 ha), K

grass production and high livestock density. The

relationship between implemented measures within

the catchments and the stream concentrations was

evaluated.

2. Materials and methods

2.1. Site description

The studied catchments are located in south-east-

ern (Rbmua and Kolstad) and south-western (Time)

Norway (Fig. 1). They represent three common

production systems in Norwegian agriculture (Table

2). In the Rbmua catchment, plant production is

dominated by cereals, and livestock density is low

(0.3 livestock units (LU) ha�1). In the Kolstad

catchment, plant production is also dominated by

cereals, while livestock density, mainly consisting of

pigs, is higher (1.0 LU ha�1) than in Rbmua. The

Time catchment has the highest livestock density (2.0

olstad (308 ha) and Time (114 ha) catchments.

Table 2

Characteristics of the Rbmua, Kolstad and Time catchments

Catchment

size, ha

% Agricultural

area

Production Livestock density,

LU ha�1

Estimated point

source, p.e. ha�1

P-AL, mg 100 g�1 Soil

Rbmua 8710 41 Cereal 0.3 0.38 8 Silt and clay

Kolstad 308 68 Cereal/pigs 1.0 0.21 14 Loam

Time 114 85 Grass/cows 2.0 0.45 17 Silty sand

Catchment size, agricultural area, dominating production, livestock units (LU), population equivalent (p.e.), P-AL (plant available phosphorus

by Ammonium-Lactate extraction) and soil texture.

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142132

LU ha�1), mainly dairy cows, in combination with

grass/forage production. The Rbmua catchment con-

sists of a plateau with steep sloping ravines. The soils

on the plateau are Stagnic Luvisol according to World

Reference Base for Soil Resources (WRB), charac-

terised by silty clay sediments (1/2–1 m deep) on

marine clay, while the ravine area is dominated by

marine clay. Land levelling of steep sloping ravines

(25% of the total arable land area) was carried out in

the 1970s and early 1980s to increase the area of

cereal production. In 1985, subsidies for land level-

ling were removed. In the Kolstad catchment, the soils

are characterised by moraine material of Cambro-

silurium origin and are classified as Endostagnic

Phaeozem in WRB. The soils in the Time catchment

developed on moraine material of Precambrium origin

(Endostagnic Umbrisol).

The mean soil P status, measured as P-AL

(Ammonium-Lactate extraction; Egner et al., 1960),

of the soils in the three catchments was 8, 14 and 17

mg P-AL 100 g�1 for Rbmua, Kolstad and Time,

respectively. The difference between catchments

corresponds to the difference in livestock density

(Table 2). In general for the region around Rbmua and

Kolstad the P-AL increased from 4.2 in 1960–1964 to

7.5 in 1983–1985. The P-AL is higher in the Kolstad

catchment, because of a higher livestock density than

average for this region. Correspondingly, the P-AL

Table 3

Measurements in the Rbmua, Kolstad and Time catchments

Discharge

measurement

Sampling No. of samples

per year

Rbmua Natural control

section

Volume proportional

mixed samples

51

Kolstad V-notch Volume proportional

mixed samples

29

Time Crump weir Volume proportional

mixed samples

29

increased from 16 in 1960–1964 to 22 in the area,

where Time is located (Krogstad, 1987). The mean

annual temperature in the two inland catchments in

south-eastern Norway, Rbmua (4.3 8C) and Kolstad

(4.2 8C) is substantially lower than in the Time

catchment on the west coast (7.4 8C). In addition,

precipitation is twice as high at Time (1154 mm) than

in the inland catchments (Rbmua: 665 mm; Kolstad:

585 mm) (Table 3). In order to enable earlier spring

cultivation, thus extending growing season, most soils

are artificially tile drained.

2.2. Catchment monitoring

The Rbmua, Kolstad and Time catchments were

monitored from 1983 to 2000, from 1985 to 2001 and

from 1985 to 2000, respectively. Monitoring of the

catchments consisted of discharge measurement,

water quality analysis and a survey of agricultural

practice within the catchments (Table 3). At the outlet

of each catchment, discharge measurements were

carried out in a cross-section of the stream by

measuring the flow depth with a pressure transducer.

Data loggers recorded flow data. The cross-section in

Rbmua was a natural control section and the flow

curve was built based on the relationship between a

series of periodic stage height measurements and their

corresponding in-stream flow measurements. In the

Source of farming

practice information

Temperature

normal, 8CPrecipitation

normal, mm

Monitoring

period

Statistical data 4.3 665 1983–1996

Questionnaire 4.2 585 1985–2000

Questionnaire 7.4 1154 1985–2001

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142 133

streams of Kolstad and Time a V-notch and a Crump

weir, respectively, were used (Table 3). The discharge

measurements during summer in the Time stream are

inaccurate because of plant growth in the stream,

which raised the water level in the Crump weir. Hence

flow data in the Time stream is not presented in this

paper. In Rbmua, flow proportional (1983–1996) and

time equidistant (1996–2000) water quality samples

were collected to form composite samples. The

different sampling strategy in different periods may

cause a systematic underestimation in the latter period

(Haraldsen and Stalnacke, 2002). Hence, for Rbmua

only the period 1983–1996 is included in this

analysis. In the Kolstad and Time streams, flow

proportional composite samples were collected

throughout the respective monitoring periods (Table

3). In all three streams flow proportional subsamples

were taken automatically and collected in a bucket,

from which a representative sample was taken after

thorough mixing about once every 2 weeks. Hence,

continuous data are available for all three steams.

Unfiltered samples were used to determine TP by

digestion with potassium persulphate and they were

analysed spectrophotometrically by the ammonium

molybdate method of Murphy and Riley (1962), with

ascorbic acid as a reducing agent. Suspended sedi-

ment was determined by filtering an exact sample

volume of 25 to 250 ml after thorough mixing

(containing at least 5 mg SS) through a pre-weighed

fibreglass filter (Whatman GF/A). Annual loads were

calculated as the cumulative sum of hourly loads

derived from hourly flow values and fortnightly

concentration values.

Information on management practice within the

catchments was derived from different sources. For the

Rbmua catchment, the source of information on

agricultural practices consists of data from Statistics

Norway for the two main municipalities in the catch-

ment. Data from Statistics Norway contain information

on soil cultivation from 1990 and onwards. In the

Kolstad and Time catchments, annual questionnaires

to farmers provided information on management

practice on each field within the catchments.

2.3. Statistical methods

The Mann–Kendall test is a standard and well-

known non-parametric method used to detect mono-

tone trends in time series of water quality data (Dietz

and Kileen, 1981). It was further developed to account

for seasonal variation, ties, missing values and

autocorrelation (Hirsch et al., 1982; Hirsch and Slack,

1984). More precisely, for a time series observed

throughout the various seasons during the study years,

the Mann–Kendall statistic for each season is defined

as the sum of all signs of differences. The seasonal

Mann–Kendall test is subsequently defined as the sum

of the Mann–Kendall statistics for all seasons. In our

study a modified version of the Mann–Kendall test

was in our study used to statistically analyse long-

term changes in SS and TP concentrations in the three

streams. Weather conditions often cause natural

fluctuation in the nutrient concentration time series,

which may impede the detection of a human induced

trend (Stalnacke and Grimvall, 2001). To account for

such fluctuations, it is essential to include explanatory

(i.e., meteorological or hydrological) variables in the

analysis if available (only for Rbmua and Kolstad).

This is accomplished by computing the conditional

distribution of the Hirsch–Slack statistic for the

response variable, given the statistic for the explan-

atory variable (Libiseller and Grimvall, 2002). Here,

this test is referred to as the Partial Hirsch–Slack

(PHS) test. This procedure can be generalised to allow

for more than one explanatory variable. In our study,

however, we only used water discharge as the

explanatory variable. Moreover, since seasonal trends

were of particular interest in our study and the fact

that the temporal behaviour may shift between years

due to the prevailing hydro meteorological situation,

we aggregated the nutrient concentrations into tertiary

data; e.g., snowmelt may occur in January or in March

depending on the hydro meteorological conditions in

a particular year. Thus, we divided the year into the

following three seasons: January–April (winter and

early spring season), May–August (growing season)

and September–December (autumn period). For each

tertiary season, flow-weighted tertiary concentrations

and water flow were calculated. Finally, the tertiary

data were used to statistically analyse time trends

using the PHS statistic (one sided test). The PHS

statistic has been shown to be powerful in situations

with monthly and quarterly data, provided there are at

least ten years of data (Libiseller and Grimvall, 2002).

Thus, it was assumed that our dataset, with tertiary

data over a 14–17 year period, was of sufficient length

Rømua

3000

3500(a)

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142134

and temporal resolution for a robust statistical analysis

of trends.

R2 = 0.9

0

500

1000

1500

2000

2500

0 1000 2000 3000 4000

SS, mg l-1

TP

, µg

l-1

Kolstad

R2 = 0.35

0

100

200

300

400

500

600

700

0 100 200 300 400

SS, mg l-1

TP

, µg

l-1

(b)

Time

R2 = 0.55

0

1000

2000

3000

4000

5000

0 50 100 150 200

SS, mg l-1

TP

, µg

l-1

(c)

µµ

µ

Fig. 2. Relationship between concentrations of total phosphorus

(TP) and suspended sediments (SS) for Rbmua (a), Kolstad (b) and

Time (c) streams. Note different scales.

3. Results and discussion

3.1. Annual concentrations of suspended sediments

and total phosphorus

Summary data on mean annual concentrations of

SS and TP for the respective monitoring periods are

given in Table 4. The Rbmua is a large scale

catchment, and hence, may show a lower connectivity

for this catchment compared to the other, smaller

catchments in this study. However, among the three

streams, the highest annual mean concentration of SS

was observed in the Rbmua stream (91 mg l�1).

Annual mean concentration of TP in the Rbmua

stream was 147 Ag l�1, although instantaneous

concentrations were much higher. The temporal

variability in SS and TP concentrations in Rbmua

showed a close relationship (r2=0.9; pb0.001) (Fig.

2a), which indicates that erosion was probably the

main process of TP transfer. Moreover, Krogstad and

Løvstad (1987) found TP contents of about 0.1% in

Norwegian cultivated soils (0–20 cm depth) and up to

0.2–0.3% after years of intensive application of P in

fertilizer. The mean annual TP/SS value (0.16%) in

Rbmua is within the range of these values, which may

also support the conclusion that agricultural soil is the

main source of SS and TP in the Rbmua stream. The

enrichment of P, described by, e.g., Øygarden (2000),

in eroded material was not reflected here. One

explanation for this could be that erosion on frozen

soil does not cause enrichment of P (Bechmann et al.,

2003). In Rbmua, the highest SS concentrations are

Table 4

Mean annual flow-weighted concentrations and loads of suspended

sediments (SS) and total phosphorus (TP) from the Rbmua, Kolstad

and Time catchments

Catchment Period Concentration Load

SS,

mg l�1

TP,

Ag l�1

SS,

kg ha�1

TP,

g ha�1

Rbmua 1983–1996 91 147 406 657

Kolstad 1985–2000 32 108 112 360

Time 1985–2001 12 195 n.d. n.d.

n.d.: no data available.

mostly observed during the winter and early spring,

i.e., January to April (Fig. 3). The TP/SS ratio,

however, showed a high temporal variability, espe-

cially at low SS concentrations, and was found to be

inversely correlated to the SS concentrations (Fig. 3).

Decreased TP/SS ratios at high SS concentration have

also been observed in other Norwegian studies

(Øygarden, 2000; Bechmann et al., 2003). This

relationship can be explained by the fact that high

SS concentrations represent erosion events with more

particles from deeper soil layers containing less P

(Øygarden, 2000). Point sources from scattered

dwellings may contribute to relatively high TP/SS

ratios in low flow/low SS situations, e.g., during

Rømua

0

2

4

6

8

10

12

14

16

1 10 100 1000

SS concentration, mg l-1

TP

/SS

rat

io

Jan.-Apr.

May-Aug.

Sept.-Dec.

Fig. 3. The enrichment of total phosphorus (TP) in relation to suspended sediments (SS) in composite samples from the Rbmua catchment.

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142 135

summer. In an earlier study in Rbmua, Wivestad

(1996) estimated that point sources of TP accounted

for only 19% of the total annual TP stream load.

Stream bank erosion is another process of TP and SS

transfer and may compensate for the elevated TP/SS

ratio by point sources, by contributing soils low in P,

resulting in TP/SS ratios in the stream similar to the

TP/SS of the soil (Krogstad and Løvstad, 1987). The

catchments, which were studied by Øygarden (2000)

in the region of Rbmua, comprised field scale studies.

These fields were neither influenced by point sources

nor stream bank erosion. They had, however, about

the same TP/SS ratio as for Rbmua (0.08–1.0%).

Thus, erosion seems to be the main process of P

transfer in the Rbmua catchment, though also point

sources may contribute to P losses.

Mean concentrations of SS (32 mg l�1) in the

Kolstad stream were low compared to the Rbmua

catchment (Table 4), despite the great differences in

size of the catchments. The correlation between TP

and SS concentrations in the Kolstad stream was

statistically significant but substantially weaker

(r2=0.35; pb0.001) than in Rbmua, suggesting that

erosion is not the dominating process causing P loss

(Fig. 2b). The homogeneous light textured and well-

structured moraine soils in the Kolstad catchment

cause high infiltration capacity and may partly explain

the relative low erosion rate. The TP mean concen-

tration in Kolstad was 108 Ag l�1, which was 2/3 of

the Rbmua concentration. The TP concentration was

relatively higher than the SS concentration in the

Kolstad stream and was 0.34% of SS. The higher

livestock density (Table 2) and use of pig slurry in the

Kolstad catchment most likely explains this differ-

ence. This is further supported by the difference in P

content of soils between the catchments (14 mg P-AL

100 g�1 soil in Kolstad and 8 mg P-AL 100 g�1 soil

in Rbmua). The process of P transfer in the Kolstad

catchment probably includes erosion, dissolution and

incidental losses.

In the Time stream, the mean concentration of SS

was 12 mg l�1, which is only 13% of the observed SS

concentration in the Rbmua stream (Table 4). The

Time catchment is dominated by grassland and gently

sloping fields with a low erosion risk. Mean concen-

tration of TP was 195 Ag l�1, which included one

event in 1993, when TP concentration of 4400 Ag l�1

and a SS concentration of 160 mg l�1 were observed.

These high concentrations were observed after spread-

ing of silage effluent on a 2 ha area followed by 19

mm of rain and may be related to either runoff from

the field or spreading very near to the stream. This

event was, however, not further investigated in field.

Generally, there is a weak relationship, though

significant, (r2=0.55, pb0.001) between TP and SS

concentrations for Time (both with and without the

outlier), indicating that erosion may not be the

dominating process of TP transfer within the catch-

ment (Fig. 2c). In the Time stream, the TP/SS was

1.6%, which is 10 times the TP/SS in Rbmua.

Correspondingly, the Time catchment also had the

highest soil P content (17 mg P-AL 100 g�1)

compared to the other two catchments (8 and 14 mg

P-AL 100 g�1) (Table 2). Data from the Time stream

showed a rather diluted influence of incidental P

losses on TP concentrations after manure application

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142136

(Withers et al., 2003). Withers et al. (2003) found,

however, obvious influence of incidental P losses at

field scale. Incidental P losses, desorption of P from

soil and erosion of soil particles with a high P content

are probably contributing to the P transfer in the Time

catchment.

3.2. Losses of suspended sediments and total

phosphorus

Losses of SS and TP were only calculated for

Rbmua and Kolstad (Table 4). Annual losses of SS

and TP (1983–1996) from the Rbmua catchment

were 406 kg ha�1 and 0.66 kg ha�1, respectively.

According to Vagstad et al. (2001), this is higher

than the losses measured in similar agricultural

catchments in Denmark and Sweden, but approx-

imately at the same level as in Finland. In the same

region as Rbmua, even higher losses of SS and TP

were reported at field scale by Øygarden (2000),

who measured SS losses up to 1892 kg ha�1 yr�1

and TP up to 1.7 kg ha�1 yr�1 in a 6-year study.

Catchment size and percentage of agricultural area

are shown to be important factors explaining P losses

(Kronvang et al., 1995; Johnes et al., 1996; Ekholm

et al., 2000), though in a study by Kronvang et al.

(2003) on 108 European catchments only 37% of the

variance was explained by runoff, percentage of

agricultural land and catchment size. Notwithstand-

ing, the size of the Rbmua catchment and the

relatively low (41%) share of agricultural land

reduce the annual losses of SS and TP compared

to losses from field scale and 100% agricultural land.

The mean annual loads from the Kolstad catchment

are 112 kg SS ha�1 and 0.36 kg TP ha�1 (Table 4),

and thus similar to the loads in Denmark and Sweden

(Vagstad et al., 2001). Low precipitation and hence

low runoff partly explain the low losses in Kolstad.

However, considering the size of catchment and share

of agricultural land the loads from Rbmua would be

expected to be lower than for Kolstad. This contrast is

mainly explained by differences in soil type between

the two catchments, giving rise to much higher

erosion risk in Rbmua than in Kolstad. The difference

in scale of these catchments also influences the

importance of point sources. While waste water

contributed to 4% of the P losses in Kolstad, the

point source contribution in Rbmua in 1994 con-

stituted 19% (Wivestad, 1996). Hence, point sources,

in addition to agricultural sources, have played a role

in the Rbmua catchment.

3.3. Annual variations in concentration and losses

Mean annual concentrations of SS and TP for the

three streams showed great variability (Fig. 4). In

Rbmua, the mean annual flow-weighted SS concen-

tration varied from 22 mg l�1 in 1996 to 232 mg l�1 in

1990. Correspondingly, the TP concentration was

lowest in 1996, with an annual mean of 56 Ag l�1, and

highest in 1986 when the concentration was 330 Agl�1. Mean annual concentrations of SS and TP were

closely related (r2=0.9; pb0.001). Weather conditions,

including the combination of temperature and precip-

itation, cause natural fluctuations in concentrations of

SS and TP (Øygarden, 2000; Ulen, 1998). However,

yearly water discharge alone did not explain the

variations in annual mean concentrations in Rbmua.

Øygarden (2000) showed that seasonal differences in

temperature have an important influence on erosion

processes, with the period of snowmelt contributing

most to erosion, and accordingly P losses, in this

region. Fig. 5 shows the annual variations in P losses

from the Rbmua catchment in winter (January–April),

spring/summer (May–August) and autumn (Septem-

ber–December). The highest P losses were measured

in winter 1990 and 1999. High P losses during

summer were found in the 1980s, while they were

relatively low during the 1990s (Fig. 5). Losses of P in

autumn were highest in 1985, 1998 and 2000, though

they were low compared to the winter.

In the Kolstad stream, highest annual flow-

weighted concentrations of SS and TP occurred in

2001, because of adverse hydro meteorological

conditions during snowmelt in April causing high

erosion. Concentrations in 1986 were also relatively

high for both SS and TP (Fig. 4c and d). In 1996,

however, the annual concentration of SS was low

while the TP concentration was high due to a very

high concentration during snowmelt. The high TP

concentration in during snowmelt could not be

explained by the known agricultural activity, hence

point sources, e.g., leakage from manure storage, may

have caused this elevated concentration of TP. The SS

concentration in Time showed low interannual vari-

ability. Similarly for the mean annual TP concen-

Rømua

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

P lo

sses

, kg

ha-1

Jan-AprMay-AugSep-Dec

Fig. 5. Losses of total phosphorus (P) from the Rbmua catchment in

tertial seasons.

Rømua Rømua

0

50

100

150

200

250

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

SS

con

cent

ratio

n, m

g l-1

SS

con

cent

ratio

n, m

g l-1

SS

con

cent

ratio

n, m

g l-1

050

100150200250300350400450500

050

100150200250300350400450500

050

100150200250300350400450500

TP c

once

ntra

tion,

µg

l-1TP

con

cent

ratio

n, µ

g l-1

TP c

once

ntra

tion,

µg

l-1

Kolstad Kolstad

0

50

100

150

200

250

0

50

100

150

200

250Time Time

(a) (b)

(c) (d)

(e) (f)

µµ

µ

Fig. 4. Mean annual concentrations of suspended sediments (SS) and total phosphorus (TP) from the Rbmua (a and b) (flow-weighted), Kolstad

(c and d) (flow-weighted) and Time catchments (e and f) (arithmetical mean).

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142 137

trations, the interannual variability is low (Fig. 4). The

extreme events and outliers are included in the

presented data to illustrate the variability, but in the

statistical trend analysis they have only very little

influence on the results.

3.4. Mitigation measures for phosphorus losses

The results and discussion presented in the sections

above demonstrate that appropriate measures to

reduce concentrations of SS and TP in agricultural

streams depend on the agricultural production system

as well as geological properties. For example the

reduced tillage is only an efficient measure where

erosion risk is high. Additionally, at the landscape

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142138

scale, in Rbmua catchment, the reduction in P losses

from point sources could also be of importance. Only

sparse data are, however, available on reductions in

point sources within this catchment. There has been a

generally high focus on point sources in this region

and Borgvang et al. (2002) estimated a 55% reduction

in P losses form municipal waste water and scattered

dwellings from 1985 to 1996 from the south-eastern

region of Norway.

In the early 1990s, diffuse agricultural sources

were included in the mitigation strategies. Different

cultivation systems, aimed at reducing soil erosion,

were introduced in Norway (Lundekvam et al., 2003).

The traditional practice of autumn tillage, which

dominated the agricultural management until the

mid-1980s, was changed. From 1990 and onwards

from 25% to 36% of the agricultural area in the

Rbmua catchment was not ploughed in the autumn

(Fig. 6). From 1993, agricultural subsidies were

targeted to areas with a high erosion risk (Lundekvam

et al., 2003) to obtain a more cost-efficient use of

subsidies. Moreover, artificial levelling in the Rbmua

catchment was carried out in 1971–1986 on an

estimated 25% of the agricultural land (Wivestad,

1996). In the years after levelling operations, high

erosion occurs through soil cracks and tile drains due

to the destabilisation of surface soils (Øygarden et al.,

1997). It may be assumed, however, that the erosion

will decrease in such areas as soil stability increases.

The removal of subsidies for land levelling reduced

these activities to zero in the late 1980s.

Rø

0

10

20

30

40

50

60

70

80

90

100

1983 1984 1985 1986 1987 1988 198

No

au

tum

n t

illag

e, %

are

a

Fig. 6. Percentage of agricultural areas in the Rbmua catchment with no ti

based on estimates by Bbrresen, personal communication.

In the areas dominated by livestock production, a

surplus of nutrients was available (Kolstad and Time)

(Fig. 7). The high P application rate (53 kg P ha�1

yr�1 in mineral fertilizer and manure) registered in

1985 in the Time catchment decreased in the

following years and then levelled off to less than 30

kg ha�1 yr�1 with minor increase in recent years (Fig.

7). The P application rate was higher in all years for

the Time catchment than for the Kolstad catchment. In

the Time catchment, however, a higher share (90–

98%) of the P was applied in the growing season

throughout the study period, whereas in the Kolstad

catchment more than 30% of the P was applied in

autumn and winter in 1988 (Fig. 8). Manure was

spread during winter in the late 1980s as well as in the

late 1990s in the Kolstad catchment, whereas in the

Time catchment manure were never spread between

1st of November and 15th of February in the study

period. The rather sparse information on P application

in the Rbmua catchment shows no trends in the use of

P (Fig. 8). No changes in number of livestock were

registered during 1985–2000 in the three catchments.

The point sources, mainly scattered dwellings,

constituted 19% of the P sources in Rbmua in 1994,

while in Kolstad and Time point sources contributed

to only 2% and 3% of P losses, respectively. No

specific data on improved wastewater treatment in

Rbmua are available, although decreases in point

source emissions are likely. For all of Norway, it has

been estimated that human point sources of P have

been reduced by 61% from 1985 to 2001 (Borgvang

mua

9 1990 1991 1992 1993 1994 1995 1996

llage in autumn. Data for 1980s are not available. Data for 1983 are

0

10

20

30

40

50

60

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

P a

pp

licat

ion

, kg

ha-1

Rømua

Kolstad

Time

Fig. 7. Mean annual phosphorus (P) application rate (mineral fertiliser and manure) in the Rbmua, Kolstad and Time catchments, 1985–2001.

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142 139

et al., 2002). In the Kolstad catchment the contribu-

tion to P loss from wastewater was reduced from 0.1

kg ha�1 yr�1 in 1988 to 0.008 kg ha�1 yr�1 in 1996.

In the Time catchment all households were connected

to tertiary treatment in municipal wastewater treat-

ment plants in 1992.

3.5. Trends

No significant changes in discharge were found

during the respective monitoring periods (Table 5).

Time series of tertiary data for concentrations of SS

and TP from the three streams were tested for

0

10

20

30

40

50

60

70

80

90

100

1985

1987

1989

1991

1993

1995

1997

1999%

P a

pp

lied

in S

epte

mb

er t

o M

arch

Kolstad

Time

Fig. 8. Phosphorus (P) application (% of annual rate) during autumn

and winter (September–March) in the Kolstad and Time catchments,

1985–2001.

significant trends during two decades (Table 5). In

general, significant downward trends ( pb0.05) were

only detected for the streams with high concentration

levels. Rbmua had the highest annual mean concen-

tration of SS, and the downward trend in annual SS

concentration was significant. Reduced autumn tillage

and other measures against erosion are likely to be the

main causes for this downward trend in SS. The stop

of the land levelling practise in the end of the 1980s

may also have had an influence on erosion in all

seasons. The significant downward trends in TP in

Rbmua is most likely to be due to decreased erosion,

Table 5

Test of significance of changes in tertiary data for water discharge

(Q), concentration of suspended sediments (SS) and total phos-

phorus (TP) and TP/SS ratio for the Rbmua catchment 1983–1996,

Kolstad catchment 1985–2001 and Time catchment 1985–2000

Catchment January–

April

May–

August

September–

December

Annually

Rbmua Q �0.357 �0.313 �0.232 �0.151

TP �0.060 �0.001 �0.002 �0.0003

SS �0.122 �0.003 �0.015 �0.0007

TP/SS 0.311 0.038 0.137 0.034

Kolstad Q �0.528 0.934 0.207 0.687

TP �0.921 �0.237 �0.411 �0.291

SS 0.012 0.953 0.291 0.127

TP/SS �0.002 �0.105 �0.016 �0.002

Time TP �0.054 0.882 �0.002 �0.025

SS �0.458 0.586 �0.216 �0.507

TP/SS �0.586 �0.182 �0.102 �0.089

Statistically significant trends ( pb0.05) marked in bold font.

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142140

since erosion is the main P transport mechanism in

this catchment. However, the trends were significant

for TP for all seasons, which may suggest that

additional factors are important. Improved wastewater

treatment may explain the reduction in P concen-

trations during summer and low flow winter periods.

The relative point source contribution has been

estimated to be around 20% in the Rbmua catchment

in 1996 (Wivestad, 1996). Using the general reduction

in TP contribution from point sources (55%) esti-

mated by Borgvang et al. (2002), this indicates that

the point source reductions can only explain a minor

part of the observed improvements in the water

quality. This is underlined by the significant reduc-

tions in SS concentration.

Trends for the Kolstad stream were difficult to

detect statistically. For the annual data, only the TP/

SS ratio showed a statistically significant trend

(Table 5). The decrease in TP/SS is a result of the

increasing SS concentration, which was significant

for the winter period. The statistical method used

here accounted for fluctuations in water discharge.

Although it cannot be ruled out whether other types

of hydro meteorological conditions have influenced

the trend test results, such as changes in snowmelt

pattern. Øygarden (2000) found that the highest

losses of SS in the Rbmua region occurred during

snowmelt. Changing climate in the Kolstad region

from stable winters to a climate with more frequent

freeze–thaw cycles may increase the number of

serious snowmelt and erosion events (Skaugen,

T.E., http://www.met.no, personal communication).

In Rbmua, serious erosion events were observed

R

0

500

1000

1500

2000

2500

3000

3500

1983

1984

1985

1987

1988

SS

co

nce

ntr

atio

n, m

g l-1

Fig. 9. Concentrations of suspended sediments (SS) in composite samples

(Fig. 9). The resulting high SS concentrations (SS

concentrations N500 mg l�1) were more frequent

early in the studied period than in recent years,

suggesting that no such effect of climate change has

impeded the detection of trends.

The concentration of SS did not reveal any trends

in the Time stream, neither on an annual basis nor for

single seasons. It should though be noted that the SS

concentrations are generally at a low level. The TP

concentrations – normally at relatively high level –

showed a significant downward annual trend (Table

5). The measures in the Time catchment has been

related to improved management of animal manure.

The reduced P application rate (Fig. 8) may explain at

least part of the decrease in TP concentration. Addi-

tionally, the incorporation of manure spread during

autumn application may have contributed to the

indications of decreased TP concentration in the

stream during autumn (significant) and winter (nearly

significant).

4. Conclusions

The main conclusions are as follows:

! Policy-induced measures resulted in a weak, but

statistically significant ( pb0.05), decrease in con-

centrations of SS and TP for catchments with high

concentration levels of SS and TP.

! Catchments with low concentration levels of SS

and TP did not show statistically significant trends

( pN0.05).

ømua

1989

1991

1992

1994

1995

1996

from the Rbmua stream. Values above 500 mg l�1 marked in black.

M. Bechmann, P. Stalnacke / Science of the Total Environment 344 (2005) 129–142 141

! The difference in concentration levels between

catchments reflected varying agricultural manage-

ment practice and geological conditions.

! The political strategies in the agricultural sector

resulted in changed management practices within

the catchments during the 1980s and 1990s.

Though changes in agricultural management prac-

tice were followed by significant decreasing trends in

concentrations of SS and TP in the streams with the

highest concentrations, improvement of measures may

be needed to further improve water quality. Focusing

measures on high-risk areas within the catchments

may improve the cost-efficiency of subsidies used to

implement measures in agricultural management

practice.

Acknowledgements

We thank the Norwegian Crop Research Institute

(Apelsvoll avd. Kise and S&rheim) and ANa(Avlbpssambandet Nordre Øyern) for providing data

from the monitoring stations, and Karl Kerner (Agro

Lingua) for careful revision of the language. In

addition, we gratefully acknowledge financial support

from SLF (Norwegian Agricultural Authority) and

SFT (Norwegian Pollution Control Authority) to the

Agricultural Environmental Monitoring Programme.

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