Journal q[ Hydrology, 128 ( 1991 ) 49-67 49

Elsevier Science Publishers B.V., Amsterdam

[2]

A new chloride leaching approach to the estimation of diffuse recharge following a change in land use

Glen R. Walker, tan D. Jolly and Peter G. C o o k

CSIRO Division qf Water Resources and Centre./~r Groundwater Studies, Private Mail Ba~ No. 2, Glen Osmond, S.A. 5064, Australia

(Received 9 November 1990: revised and accepted 15 January 1991)

ABSTRACT

Walker, G.R., Jolly, I.D. and Cook, P.G., 1991. A new chloride leaching approach to the estimation of diffuse recharge following a change in land use. J. Hydrol., 128:49 67.

A new approach has been developed to estimate the increase in groundwater recharge following land-use modification. The approach uses the degree of leaching of chloride to quantify soilwater drainage below the root zone which ultimately leads to groundwater recharge. It is more general than similar previously reported techniques for analysing transient chloride profiles to infer recharge rates, and hence has wider application. We have applied the technique to a field situation in southern Australia where clearing of native vegetation for agricultural production leads to large increases in groundwater recharge. The examples serve to demonstrate the technique and some of the practical difficulties in the application of solutes techniques to recharge estimation.

I N T R O D U C T I O N

In a recent review of recharge estimation in arid regions, Gee and Hillel (1988) concluded that techniques based on conventional water-balance methods are likely to yield large errors. This is because most approaches rely on estimates of evapotranspiration, the errors of which usually exceed the magnitude of the recharge flux being estimated. Also, in areas where water tables are deep, piezometric techniques are inapplicable because the unsaturated zone buffers the response to seasonal rainfall. Furthermore, where recharge rates are low, insufficient displacement of artificial tracers may have occurred over the period of measurement to give useful results. It is for these reasons that much of the recent effort on recharge estimation in arid and semi-arid regions has focussed on the use of environmental tracers (Allison, 1987).

Two radioactive environmental tracers which have been used are tritium and 36C1; their natural concentrations in the atmosphere and soil were greatly enhanced by nuclear bomb testing in the 1950s and 1960s. However, where

0022-1694/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved

50

N O T A T I O N

G . R . W A L K E R ET A L

C(z) chloride concentrat ion at depth z [M L 3] Cb chloride concentration at depth zu [M L 3] C. new equilibrium chloride concentration [M L 3] D deep drainage rate below root zone [L T- ~] Qb total amount of water moving below depth Zb [L] Qd total amount of water moving below depth z d [L] Qr total amount of recharge to unconfined aquifer [L] R recharge rate to unconfined aquifer [LT ~] t time [T] =b depth of base of the zone under consideration [L] zcf depth of the chloride front [L] zpf depth of the pressure front [L] zr depth of the root zone [L]

Greek

A O(z) 0

letters difference in quantity at depth z for two different land uses difference in quanti ty for two different land uses water content at depth z [L 3 L -3] average water content between zr and Zpf [L3L 3]

Spatial relationships in the soil profile

s I i1~ ,

Deep drainage rate i ~

D = dQa/dt [ ) ' ~ t

soil surface z = 0

depth of rooting z = z r

. . . . . . . . . . . . . . . . depth of choride front z = zcf

. . . . . . . . . . . . . . . . depth of pressure front z = z¢

dQb/dt base of zone under consideration Z ~ Z b

Recharge rate R = dQ~/dt

top of unconfined aquifer

ESTIMATING DIFFUSE RECHARGE: A CHLORIDE LEACHING APPROACH 51

recharge rates are low, these tracers may not have been displaced far below the plant root zone, so there may also be some difficulty in converting apparent fluxes to recharge rates.

Another commonly used environmental tracer is the chloride ion, which is highly soluble, non-absorbing, chemically conservative, and easily measurable. It has been used for dryland recharge estimation over the last two decades (e.g. Ericksson and Khunakasem, 1969; Allison and Hughes, 1978; Peck et al., 1981; Sharma and Hughes, 1985). In addition, the leaching of salt from the root zone has been used to estimate drainage fluxes in irrigation areas for over 30 years (e.g. USSL, 1954). All these techniques assume 1-D vertical movement of water and salt (i.e. no lateral flow). Most of these studies rely on the assumption of steady-state chloride fluxes in which the chloride input via rainfall, dryfall and irrigation is equal to the chloride output below the root zone (for dryland conditions see Allison and Hughes, 1978, 1983; Peck et al., 1981; Sharma and Hughes, 1985; Allison et al., 1985; Sukhija et al., 1988: Dettinger, 1988: for irrigated conditions see United States Salinity Laboratory (USSL), 1954; Frenkel, 1984; Oster, 1984; Shalhevet, 1984; Ayres and Westcott, 1985; Shaw and Thorburn, 1985).

Following a change in land use it may take considerable time for steady- state conditions to be re-established with respect to chloride. Until the new equilibrium occurs, the steady-state technique is invalid and a transient approach has to be adopted. To our knowledge, two approaches based on transient analysis of chloride profiles for recharge estimation exist, those of Rose et al. (1979) and Allison and Hughes (1983). The approach of Rose et al., known as SODICS, is based on the mass balance of chloride in the root zone. The concentration of the drainage flux is assumed to be proportional to the mean chloride concentration in the root zone. Whether such an approxi- mation is valid needs to be tested for each individual situation (as done by Thorburn et al., 1990). The approach of Allison and Hughes, known as the chloride front displacement method, relies on observations of the movement of a particular chloride pattern with depth which retains its shape during the leaching process. The degree of movement of the pattern is used to infer the rate of movement of water. A problem with this technique is that the chloride profile can become distorted during leaching owing to non-piston flow mechanisms, making the technique difficult to apply.

In this paper we develop a generalisation of the chloride front displacement technique. It retains the essential features of the original technique in that it uses the leaching of a particular chloride profile to infer rates of movement of water in the soil. However, by using an integrated water and chloride mass- balance approach, it avoids the need to assume piston flow and remains valid even if the chloride profile becomes distorted. It also avoids the need to make

5 2 G . R . W A L K E R E T A L .

an assumption similar to that made in the SODICS model about the chloride concentration of the soil water draining from the root zone.

The chloride front displacement method is representative of a range of solute techniques which rely on the assumption of 1-D flow, and more importantly, on piston flow. The effects of anion exclusion (Bond et al., 1982), aggregate dispersion (Passioura, 1971), preferred pathways (Germann et al., 1984) and general dispersion (Rose and Passioura, 1971) mean that this assumption is unlikely to hold absolutely. Yet, experience suggests that many solute techniques are still sufficiently accurate for use in many circumstances (e.g. Zimmermann et al., 1967). This indicates that it is important to develop formulations, such as those given in this paper, that do not rely on the assumption of piston flow yet retain the important characteristics of the original techniques. This enables one to deal with some discrepancies due to non-piston flow effects.

Although solute profiles may be used to estimate soilwater fluxes, they cannot be used by themselves to estimate the time delays for changes in land use to affect groundwater recharge. Jolly et al. (1989) showed that a change in land use which increased deep drainage led to a downward-moving pressure front. Above the pressure front, the soilwater content and fluxes of water reflect the new land use, whereas below it, such parameters reflect the old land use. When this pressure front reaches the water table, groundwater recharge changes. In many situations, the pressure front may be too deep for practical sampling. However, the position of the chloride front may be used to estimate the position of the pressure front if a relationship for the relative rates of movement of solute and pressure fronts can be developed. This has been done for a homogeneous profile by assuming piston flow (Warrick et al., 1971; Raats, 1984). In this paper, we show similar relationships for more general conditions.

THEORY

For the purposes of this paper it is assumed that a change in land use leading to increased vertical soilwater fluxes has taken place some years earlier. It is further assumed that the flow is 1-D and in the vertical direction.

Estimation of the increase in soilwater flux by water balance alone

Mass balance of water in the zone between the bottom of the root zone under the new land use, Zr, and some depth, Zb, is given by

• 7. b

AQd - AQb = _1 ~O(z)dz (1) 2r

ESTIMATING DIFFUSE RECHARGE: A C H L O R I D E LEACHING APPROACH 53

where AQd is the increase in the total vertical drainage below the root zone, and AQb is the increase in total amount of water moving below z b (the base of the zone being considered), both of which result from the years of changed land use. 30(z) is the difference in volumetric soil water content at depth z, resulting from the change of land use. AQd is therefore the changed input to the zone under consideration, AQb is the changed output and the right-hand side of eqn. (1) is the change in storage.

Jolly et al. (1989) showed that if a change of land use results in increased drainage below the root zone, a zone of increased water content will form in the upper soil profile bounded below by a downward-moving pressure front. Below the pressure front, AQb and 60 are zero and (1) becomes

2pf

AQd = I gO(z) dz (2) =r

where Zpf is the depth of the pressure front under the new land use. When the pressure front reaches the water table there is an increase in recharge to the groundwater. Once this happens, hydraulic equilibrium associated with the new land use implies that the drainage flux, D (= dQd/dt ), is equal to the new recharge rate, R (= dQr/dt).

Mass balance of chloride

For the following we assume that the input of chloride to the soil profile does not change with time. Under steady-state conditions we adopt the conceptual model of Gardner (1967), where chloride concentration increases more-or-less monotonically through the root zone owing to plant uptake of water. At the base of the root zone the chloride concentration becomes relatively constant and remains so for some depth. Under steady-state con- ditions this constant chloride concentration, Cb, is equal to the input of chloride (by rainfall plus dryfall) divided by the deep drainage rate. Following a change in land use, in which the deep drainage increases, leaching of the soil chloride occurs. A mass-balance argument for chloride, similar to that given for the soil water, can be used. We assume that there exists some depth, z~, at which the chloride concentration remains at the value of Cb. This implies

2b

AQbC b -- ~ 6[O(z)C(z)]dz (3) o

where 6[O(z)C(z)] is the difference in chloride storage at depth z, resulting from the change in land use, C(z) being the soilwater chloride concentration at depth z. The term on the left-hand side represents the total change in chloride

54 G.R. WALKER ET AL.

output from the zone under consideration, and the right-hand side represents the change in chloride storage within that zone. For brevity we omit the explicit dependence of C and 0 on z from here onwards.

If z b is chosen to be Zpr (or greater), AQb should be negligible and eqn. (3) implies that the total amount of chloride in the soil profile to the depth of the pressure front under the new land use should be the same as that to the same depth under the previous land use.

Integration o f the water and chloride balances to estimate soilwater f lux

Using eqn. (1) to substitute for Z~Qb in eqn. (3) we obtain

2h 2 r

= i 310(1 - C/Cb)]dz - i 60dz (4) AQd 0 0

To compare eqn. (4) with the chloride front displacement method of Allison and Hughes (1983), we need to define a generalised depth of chloride front, Zcr, as follows:

2Cl 2b

I Odz = | O(C b - - C)/(C b - - C n ) d z ( S )

0 0

where C~ is the new equilibrium value of chloride concentration given by

C, = CbD°/D" ~ CbQ~/Q~ (6)

and where the superscripts o and n refer to the old and new land uses respectively. Q~ is the total drainage that has occurred since the change in land use and Q~ is the drainage that would have occurred over the same time period if the land use had not changed. If the chloride front is sharply defined (i.e. the chloride concentration profile resembles a step function), it is clear that Zcr as defined in eqn. (5) is the chloride front. Where the chloride front is more diffuse, the definition of a chloride front becomes less obvious, but the above definition appears suitable.

Using the above definitions, eqn. (4) transforms to

Left-hand side = AQd = Q ~ - Q~ = Q~(C b - - C n ) / C b (7a)

Zb Zr

Right-hand side = ~ 6 [ 0 ( C b - C)/Cb]dZ - I 60dz 0 0

[(; ) = A 0dz (Cb -- Cn)/Cb - 60dz (7b) 0 0

ESTIMATING DIFFUSE RECHARGE: A CHLORIDE LEACHING APPROACH 55

leading to

Q~ = A Odz - 6 0 d z ' - - C n / C b ) (7c) o o

where the A notation refers to the change in the square-bracketed term with change in land use. Note that Zcf changes as a result of the change in land use and so eqn. (7c) can be decomposed into the following:

Q~ = 0 dz + 60 dz + 60 dz C n / C b (8) 2c°1. 2 r 0

The second and third terms in eqn. (8) will remain constant if the water content above the pressure front remains constant. Equation (8) describes a generalised chloride front displacement method. Differentiating eqn. (8) with respect to time will lead to an estimate of deep drainage rate which is indepen- dent of the second and third terms as they are constant.

To compare the relative rates of movement of the pressure and chloride fronts, eqn. (8) together with eqn. (7a) is used to substitute AQd in eqn. (2) and then the resulting expression is differentiated with respect to time to yield

dzcr _ •0(Zpf) (9) dzpr 0(zcf)(1 - C,/Cb)

Under the assumption of C, ,~ Cb, which is the case when R n ,> R °, eqn. (9) is the same as eqn. (26) of Raats (1984), which was developed for a homo- geneous soil profile under a similar assumption.

F I E L D E X A M P L E S

To illustrate the technique we show two examples from southern Australia in which groundwater recharge increases owing to the clearing of native vegetation for agricultural production. Under the native Eucalyptus vegetation found in southern Australia, recharge rates are extremely low (< 0 .2mm year 1 ; Allison et al., 1990) leading to a high concentration of chloride in the soil profile where the water table is deep. This gives rise to a characteristic chloride profile in which the chloride concentration increases from low values at the soil surface to concentrations of the order 5000-20 000 mg 1-~ and then remains relatively constant for considerable depth. Following clearing, recharge increases significantly (up to two orders of magnitude; Allison et al., 1990) resulting in leaching of the stored chloride.

The first example deals with a situation where a suitable native vegetation 'control' site exists. By this we mean that there is a native vegetation site where

56 G.R. WALKER ET AL.

the current vegetation and soil conditions are believed to match those which existed at the site of interest prior to clearing. In this case, which is the ideal situation, the theory is applied in a straightforward manner.

In the second example, data from a number of core holes drilled beneath cleared conditions are presented. In all these cases it is not possible to locate a suitable native vegetation 'control' site and so a procedure is developed to avoid this problem. Although it is specific to this particular field site, the procedure highlights the approach which must typically be taken when dealing with 'real world' sites.

In both examples, the procedure for estimating the increase in drainage is as follows.

(1) Estimate the position of the chloride front before the change in land use. This provides a 'marker' which is then used to trace the downwards movement of soil water. As discussed above, this can be rather difficult in many circumstances.

(2) Estimate the position of the chloride front below the new land use (eqn. 5).

(3) Calculate the increase in total drainage below the root zone using eqn. (8). In both examples, where we partially validate the theory, the increase in total drainage is also calculated using eqn. (2).

Site description and methods

The data presented in both examples are from a site located near Kulkami (latitude 35°9'S, longitude 140°17'E) in the South Australian portion of the Murray Basin (Fig. 1). A detailed description of the site and the methods used is given by Jolly et al. (1989). Briefly, the climate is semi-arid with the majority of the rainfall (mean of 370mmyear -~) occurring in April-October. The chloride concentration in rainfall and dryfall measured at a nearby site is 4 mg 1-~ (Blackburn and McLeod, 1983). The mean Class A Pan evaporation is approximately 1800mmyear -1. Unconfined ground water is found at a depth of 50-70 m, and has a salinity of approximately 2000-3000 mg 1-1. The native mallee vegetation (Eucalyptus spp.) at the site has been progressively cleared over the last 25 years, leading to fields with a range of ages since clearing. Following clearing, dryland crops and pastures have been grown. An extensive area of native vegetation remains adjacent to the site.

Sampling to obtain depth profiles of gravimetric water content (determined by oven drying), chloride concentration in the soil water (determined color- imetrically; Taras et al., 1975), matric suction (estimated using the filter paper method; Greacen et al., 1989) and particle size compositions (determined using the pipette method; Lewis, 1983) was carried out in several of the cleared

ESTIMATING DIFFUSE RECHARGE: A CHLORIDE LEACHING APPROACH 57

i

e

R~ h m ~ r ~~L....41Loxton

0 25km

oKulkamm Fig. I. Location of the study site.

fields and in the nearby native vegetation. Seven holes were drilled in the native vegetation and five in the adjacent cleared fields.

R E S U L T S

Example I

Profiles of gravimetric water content, chloride concentration, matric suction and percentage clay with depth for holes under native Eucalyptus vegetation and cleared conditions, respectively, are shown in Figs. 2 and 3. These holes are typical of the conditions at this site and are part of a more extensive data set given by Jolly et al. (1990). In the case of the native vegetation hole, the gravimetric water contents remain relatively constant below 2 m, averaging 5% (ignoring the root zone which had dried out over summer). The gravimetric water content of the cleared site averages 7% between 2 m and the well-defined pressure front (Zp0 at 10.5 m. The average soil matric suction below 2 m is 4600 kPa for the site under native vegetation, whereas for the site under cleared conditions, the mean soil matric suction is 70kPa beween 2 and 10.5m and 1000kPa below 10.5m. A possible expla- nation for the discrepancy in matric suction between the two sites below 10.5 m is that the pressure front is not an exact boundary and that a small amount of post-clearing water has moved below this depth. As only a small amount of water is required to lower the matric suction to this degree, its effect on recharge estimation is negligible and can be ignored.

5 8 G.R. W A L K E R E T AL.

c~

2 ,

4 •

6.

8

1 0

12

14

16

18

2 0 I I I I I

0 . 0 5 0 . 1 0 0 . 1 5

GRAVIMETRIC W A T E R

C O N T E N T ( g / g )

O ,

2t

4.1

6.1

8~

I0~

12~

14 .

16 .

18 .

2O I I I I I I I I I I T I I I I ~ I i 3 6 g 12 15 0 2 4 6 8 10

( T h o u s a n d s ) ( T h o u s a n d s ) CHLORIDE ( m R / I ) M A T R I C SUCTION ( k P a )

OI

16 ̧

18 ̧

.~0 i i 112 i i i 0 4 8 16 2 0 2 4 2 8

% C L A Y

Fig. 2. Gravimetric water content, chloride in soil water, matric suction and percentage clay profiles of a core hole drilled beneath native Eucalyptus vegetation.

0

2

4

6

8

10

12

14

16

18

2 0 I r I I I 0 . 0 5 0 . 1 0 0 . 1 5

GRAVIMETRIC W A T E R

C O N T E N T ( g / g )

8t

8~

I i 24

144

16 ,

18~

!01 = i = = = i r i = 3 6 9 12 15

( T h o u s a n d s ) CHLORIDE ( m g / l )

0

2

4

6

8

I 0

12

14

16

18

]0

0

2 •

4"

:8.

8"

i i , , ~ ; r i i : 0

2 4 6 8 10

( T h o u s a n d s ) MATRIC SUCTION ( k P a )

4~

6~

8"

O'

r i / i i i 4 8 12 1 6 2 0 2 4 2 8

CLAY

Fig. 3. Gravimetric water content, chloride in soil water, matric suction and percentage clay profiles of a core hole drilled in a field cleared of native Eucalyptus vegetation 12 years ago.

ESTIMATING DIFFUSE RECHARGE: A CHLORIDE LEACHING APPROACH 59

Chloride concentrations in the soil solution under native vegetation increase from approximately 4400 mg 1 ~ at the soil surface to a mean of 8400 mg 1 at depths greater than 5 m. In the case of the cleared site, chloride concen- trations increase from approximately 1900 mg 1- ~ at the soil surface to a mean of 8200 mgl ~ below 8.0 m. The similarity in both the values of Cb and the percentage clay profiles suggest that the native vegetation site is a good 'control ' for the site beneath the cleared field.

Equation (5) was used to calculate Zcf for both the native vegetation (z°O and cleared sites (z~0. The integrations were performed by trapezoidal approximation on the sampling interval (usually 0.5 m) and were carried out on a personal computer using a spreadsheet package. The values of z°f and ~n "c f

obtained were 1.25 and 5.25 m, respectively. On visual inspection these values appear reasonable. Using these values of zcr, together with the water content data from the profile under cleared conditions, Qd was estimated, using eqn. (8), to be 350mm. In addition, Qd was estimated from the water content data of both holes, using eqn. (2), to be 320 mm. Note that we have assumed that the ratio Q/Cb is approximately zero and have added the term

£r

f 6Odz (10) 0

to both estimates to avoid complications when selecting the depth of the root zone. The agreement between the estimates of Q~ is considered to be good. The small discrepancy can be attributed to the fact that although the sites are located close to each other in the same landscape unit, they are still not an exact pair of pre- and post-clearing sites. This can be seen by examining the total amount of chloride to Zpf for the two sites: 390 mg cm -2 in the case of the native vegetation site, and 520 mg cm 2 in the case of the site beneath cleared conditions. These data should be identical for exactly matched sites.

Example 2

In this example we present data from five holes drilled beneath cleared conditions at the site, including the hole presented in Example 1 (KUL11). Despite the fact that seven holes were drilled beneath native vegetation at this site, none were suitable as 'controls' for any of the five cleared holes presented here (except for KUL11, as described in Example 1) and so we have sought an alternative approach to the application of the theory. The approach developed here is site specific but highlights the way in which the problems encountered in a typical field situation can be overcome.

From Fig. 2 it can be seen that the matric suctions under native vegetation are extremely high owing to the efficient water extraction of the Eucalyptus

60 G.R, W A L K E R ET AL.

vegetation. At such high suctions, small changes in water content are associated with extremely large changes in matric suction. This suggests that the soilwater content under native vegetation should depend mainly on the particle size distribution. In particular, the water content should depend on the percentage clay because, at these high matric suctions, all the soil water is stored in the clay size pores. F rom all the samples collected beneath native vegetation at the site where particle size distribution has been determined (including those from some of the holes not reported here), there exists a reasonable correlation between gravimetric water content (g g- ~ ) and percentage clay:

0g = 0.0034(%clay) + 0.010 (r 2 = 0.59) (11)

F rom this relationship it is possible to construct the pre-clearing water content profile for a cleared hole using its percentage clay profile.

To use the generalised chloride front displacement approach (eqn. 8) properly, it is necessary to examine the natural variability in the chloride front position under native vegetation, as well as the factors influencing this position. One of the principal influences appears to be near-surface soil texture (Fig. 4). For the data shown in Fig. 4, the position of the chloride front was objectively determined using eqn. (5). It is unfor tunate that our data set does not include any native vegetation holes with mean percentage clay in the top 2 m of between 5 and 13%. However, this does not unduly affect subsequent calcu- lations, where we use the correlation

. . . . 0.21(%clay) + 3.86, %clay ~< 20 (r 2 0.88) (12) - C f - -

~o and where -ct (m) is the posit ion of the chloride front under native vegetation. The chloride profiles of the cleared holes are similar in form to those

from beneath native vegetation, except that they show various degrees of leaching in the top 5 m. The values of Cb for each of the cleared holes are presented in Table 1. Note also that the chloride concentrat ions in the root zone at the cleared sites (C,~) are much less than the values of Cb, and this can be used to simplify eqns. (3)-(8). The estimates of -or'° (determined using eqn. 12) and ,n (calculated using eqn. 5) for the cleared holes are shown in =of

Table 1, as is the value of Zpr for each hole, determined from its matric suction profile.

Qd was calculated using eqn. (8) for each of the cleared holes and is shown in Table 1 (labelled integrated approach). The terms z°r and z2f were determined as described above. 60 was estimated using the gravimetric water content, as measured for the cleared hole, and estimated for the native vegetation using the correlation given in eqn. (11). This avoids any specific matching of native vegetation and cleared profiles as was carried out in earlier papers (Cook

E S T I M A T I N G D I F F U S E R E C H A R G E : A C H L O R I D E L E A C H I N G A P P R O A C H 61

4.0

3 . 5

3 . 0

2.5

° F z.o N

1 .5

1.0

0 5

Z o : - 0 . 2 1 % CLAY * 3 . 8 6 c f

2 r : 0 . 8 8

I 2 20

I I I I I I I I 4 6 8 1 0 1 2 1 4 1 6 1 8

% CLAY

Fig. 4. Plot of percentage sand in the surface 2 m vs. depth of the chloride peak under the native vegetation holes at the site.

TABLE 1

Summary of measured and estimated data for the holes beneath cleared conditions

Hole Ca .o n -of Zcr zpf Qd Qd (mgl i) (m) (m) (m) Water balance Integrated

approach approach (mm) (mm)

. . . . . ,30 ,Zcf - - ,Zcl.

Z p f - - Z r 0

KUL07 10700 1.6 6.1 13.0 560 590 0.38 0.37 KUL08 10800 2.0 4.3 10.5 590 590 0.24 0.26 KUL11 8200 1.4 5.3 10.5 370 320 0.41 0.47 KUL15 15850 0.0 2.5 8.5 240 300 0.33 0.28 KULI9 13750 0.1 1.3 5.5 220 150 0.27 0.36

Values of z~f were estimated using the correlation given in eqn, (11). Values of z2l- were calculated using eqn. (5). Values of Qd for the water balance approach were calculated using a modified form ofeqn. (2). Values of Qd for the integrated approach were calculated using a modified form of eqn. (8). Values of 60/0 were calculated using eqns. (14) and (15).

62 G.R. WALKER ET AL,

et al., 1989; Allison et al., 1990) which would be subject to spatial variability of soil properties.

As a check on the consistency of the theory, as applied to this particular field situation, eqn. (2) is also used to calculate Qd for each of the cleared holes (labelled water balance approach in Table 1). A comparison of the estimates obtained from eqns. (2) and (8) is shown in Fig. 5. Note that, as in Example 1, we have assumed that the ratio Cn/Cb is approximately zero and have added the term given by eqn. (10) from both sets of estimates to avoid complications when selecting the depth of the root zone. The r 2 of these data is 0.92 and the agreement is considered good given the errors inherent in the application of both techniques.

Another check on the consistency of the theory can be achieved by using eqn. (9), where the ratio of the rates of movement of the chloride and pressure fronts is equated to the ratio offO(zvO to 0(Zc0. For the purposes of this check, the following alternative formulation is used:

,n .o 60 ~"cf - - ~c f - (13) Epf - - 2 r 0

E

0 _J

m

8°° I 700

600

500

400

300

200

r ~ - O 9 2

/

Y:×

t I ! P [ I 200 400 600 800

Od ESTIMATED FROM WATER BALANCE AND CHLORIDE LEACHING (mm)

Fig. 5. A comparison of the estimates of Qa, as calculated using a water-balance approach (eqn, 2) and using the generalised chloride front displacement technique (eqn. 8),

ESTIMATING DIFFUSE RECHARGE: A CHLORIDE LEACHING APPROACH 63

where

60 =

and

(!f Odz)/ zpf zr) (14)

(s )/ 0 = Odz ( z~ f - z°O (15)

"cf

These ratios were calculated assuming a root zone depth (Zr) of 1.0 m and are shown in Table 1. The ratios are in reasonable agreement for each of the cleared holes and have a similar relationship to that shown in Fig. 5, but with an r z of 0.58 which is heavily influenced by hole KUL19.

In the formulation of the theory, it was assumed that the chloride concen- tration of the drainage water at depth z b was the same as that in the total soil water at that depth. This would not be the case if there was significant preferred pathway flow or anion exclusion effects at depth Zb. The fact that four of the five data points in Fig. 5 lie close to a 1 : 1 line suggests that these effects are negligible at this field site.

Conversion to a recharge rate

Although eqn. (8) enables calculation of the increase in total deep drainage, the quantity normally of interest is the mean annual recharge rate (R - D). To calculate actual recharge rates, a knowledge of the water content and chloride profiles at a number of times following land-use change is required. In our case this was not possible owing to the time scale (decades) for substantial leaching to occur. Instead we compare water content and chloride profiles from beneath native vegetation with those from beneath adjacent cleared fields. To convert the estimated increases in total deep drainage to a new equilibrium recharge rate we simply divide Qd by the number of years since the fields were cleared. This leads to a possible error in that it implicitly assumes that Qd is zero at the time of the change in land use and increases more-or-less linearly with time. It is possible that there is a large initial increase in deep drainage (e.g. Thorburn et al., 1991) which invalidates the assumption of approximate linearity. However, as the time since clearing increases, the relative error associated with this uncertainty will decrease.

64 G.R. WALKER ET AL.

DISCUSSION AND CONCLUSION

In this paper, we show both theoretically and with the use of two field examples how soil chloride profiles may be used to estimate increased rates of groundwater recharge, which often accompany a change in land use. Our method retains the essential features of a previously reported technique (referred to here as the chloride front displacement technique; Allison and Hughes, 1983), in that it uses the leaching of the chloride profile to infer rates of movement of water in the soil. However, the chloride front displacement technique was not mathematically defined and implicitly assumed piston flow. It relied on the chloride front being sharply defined and gave no indication of how to select the position of this front when it is more diffuse. The integrated water and chloride mass-balance approach developed here (which we refer to as the generalised chloride front displacement technique) avoids the assumption of piston flow and remains valid even if the chloride profile becomes distorted.

Another model for using transient soil chloride profiles to infer water flow is that known as SODICS (Rose et al., 1979). This approach uses the changes in mass balance of chloride in the root zone to estimate drainage fluxes of water below the root zone. It makes the assumption that the chloride concen- tration of the drainage water is proportional to the mean concentration in the root zone. This assumption is difficult to test because of the amount of spatial and temporal data required (e.g. Thorburn et al., 1990). The generalised chloride front displacement method avoids making such an assumption.

Although the formulation of our approach uses water and chloride bal- ances in the soil profile, we use eqn. (8) in preference to eqn. (4) to estimate recharge as it does not explicitly use mass balances. This minimises problems associated with spatial variability of soil texture which can greatly influence the mass balances. In practice, mass balances can be used where a single site can be sequentially sampled through time, preferably with several holes each time. This was done with the SODICS model (Thorburn et al., 1990, 1991); for our situation such sampling was not practical.

Spatial variability is still a concern in both chloride front displacement methods. In the first field example given here a chloride profile under the new land use was compared with a profile under the old land use in an adjacent field. If the chloride front under the new land use (z~f) is not significantly different from that under the old land use (Zc°f) the spatial variability of -° ~cf

becomes important, and so a method of choosing profiles with which to compare is required. In our second example we were able to use the corre- lation of ~cf~° with percentage clay in the top 2 m to avoid matching profiles. This may be difficult in other circumstances and so other approaches may need to be adopted.

ESTIMATING DIFFUSE RECHARGE: A CHLORIDE LEACHING APPROACH 65

A limitation which is recognised but not addressed in this paper is the procedure for converting our measured deep drainage rates to recharge. In the absence of data, we assume that to convert the estimated increases in total deep drainage to a mean annual recharge rate under the new land use, we simply divide Qa by the number of years since the fields were cleared. In arid and semi-arid regions it is expected that extreme climatic variability results in considerable temporal variability in the deep drainage flux. The first significant drainage fluxes under the new land use will be dependent on both climatic conditions and the nature of the land use change (Cook and Walker, 1990; Thorburn et al., 1991). The mean annual recharge rate may change as the new vegetation establishes or is modified. The method by which our approach is applied to the field situation does not enable us to discriminate between these effects.

A change in land use that leads to increased drainage also causes a pressure front to occur. An important component of the work presented here is to esti- mate the relative rates of movement of the chloride and pressure fronts. By knowing the position of the chloride front, one can then estimate the position of the pressure front. It is only when the pressure front reaches the water table that a change in recharge occurs. Thus, the above calculation enables us to estimate time delays before the effect of the change in land use reaches the water table.

Finally, a limitation that is characteristic of all solute profile techniques is that they provide only point estimates of recharge, which may not necessarily reflect the areal recharge regime (Cook et al., 1989).

ACKNOWLEDGEM ENTS

This work was made possible by the financial assistance of the Australian Water Research Advisory Council Grant No. 86/05. Glen Walker acknowl- edges the support provided by the French Government for his stay at Universit6 de Paris Sud, during which part of the work was carried out. Peter Cook is supported by an Australian Postgraduate Priority Research Award and a Centre for Groundwater Studies Scholarship. Thanks also to Drs. Munna Sharma, Colin Johnson and Chris Barnes for useful criticism of the manuscript. Messrs. Andrew Holub and Mark Trenordan provided valuable field and laboratory assistance.

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