Evaluation of soil water storage efficiency for rainfall harvesting on hillslopemicro-basins built using time domain reflectometry measurements

M. Previati *, I. Bevilacqua, D. Canone, S. Ferraris, R. Haverkamp

Department of Agricultural, Forestry and Environmental Economics and Engineering (DEIAFA), University of Turin, via Leonardo da Vinci 44, 10095 Grugliasco (TO), Italy

Agricultural Water Management 97 (2010) 449–456

A R T I C L E I N F O

Article history:

Received 2 July 2009

Accepted 8 November 2009

Keywords:

Water harvesting

Water conservation

TDR

Semi-arid areas

Micro-basins

A B S T R A C T

Micro-basins are slope management structures built out of earth and stones on hillslopes around

cultivated trees (e.g., olive trees) for the harvesting of rainfall and runoff water, and for the rehabilitation

of land degraded by water erosion.

In this study, the results of an experimental survey for the comparison of soil water content for both

inside and outside the micro-basins are analyzed. Measurements are taken after some rainfall events

from January to December 2003 in a hilly region of Central Tunisia. The time domain reflectometry

technique is used to measure soil moisture in 15 sets of soil profiles (inside and outside) at three different

depths. Four different soils are evaluated, i.e., Cambisols, Kastanozems, Arenosols, and Calcisols.

The data analysis shows a significant improvement on the water stock obtained by this type of

management. The differences in water storage with respect to soil type, depths, and tillage are evident,

but strongly connected to farm management. For optimal management conditions an important increase

of average water stock is observed; however, for bad or no farm management the amelioration is zero or

is even deteriorating the state of vegetation.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

The hydrological cycle in soil begins with the entry of water byinfiltration, then continues with a temporary storage of water inthe soil, and ends with its removal from the soil by drainage,evaporation, and plant uptake. Infiltration determines how muchwater enters the root zone and how much will runoff. Runoff isconsidered as the portion of water that exceeds the infiltrationcapacity. Hence, the rate of infiltration affects not only the watereconomy of vegetation, but also the potential amount of surfacerunoff and the risk of soil erosion in semi-arid zone cultivatedhillslopes (Senay and Verdin, 2004; Schiettecatte et al., 2005;Oweis and Hachum, 2006).

Water harvesting systems for runoff water collection andstorage represent an attractive solution for resolving waterscarcity in various parts of the world (e.g., Tabor, 1995; VanWesemael et al., 1998; Oweis and Hachum, 2006; Schiettecatteet al., 2005; Frot et al., 2008). According to the Food andAgriculture Organisation (FAO) of the United Nations (2004),these methodologies can be useful in arid and semi-arid areas(such as the southern Mediterranean regions), which are often

* Corresponding author. Fax: +39 011 6708619.

E-mail address: maurizio.previati@unito.it (M. Previati).

0378-3774/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.agwat.2009.11.004

characterized by irregular distribution over space and time of thewater resources.

Historically speaking, agriculture using surface runoff and rainharvesting techniques was extensively practiced as early asthousands of years ago. As recently reviewed by Vohland andBarry (2009), in these arid and semi-arid zones, a huge variety oftraditional as well as innovative in situ rainwater harvestingpractices exist. Typical in situ structures used on slope manage-ment are linear structures (e.g., embankment of stone and/or earth,or grass strips) which can be sophisticated, such as the teras systemin Sudan (van Dijk, 1997) and terracing as practiced in East Africa.Semi-circular bunds are common in the arid and semi-arid zones ofNorthern Africa and Sahel (e.g., micro-basins or half moons or demi

lunes (Barry and Sonou, 2003)). Pitting cultivation is practiced indifferent forms which are known as Zai in Burkina Faso (Kabore,1995; Kassogue et al., 1996; Ouedraogo and Kabore, 1996;Fatondji et al., 2001; Kabore and Reij, 2004), Tassa in Niger(Hassan, 1996), or the Chololo pits and Ngoro pits for the Matengopeople in East Africa (Critchley and Mutunga, 2003; Kato, 2001;Mati and Lange, 2003; Malley et al., 2004). These various systemsdiffer mainly in the size of the pits. In general, biomass productionis improved by applying a layer of mulch in the pit before planting.Other approaches, such as conservation tillage, improve infiltra-tion ability on the field scale (Stroosnijder, 2003).

Micro-basin watershed management is realized by little wallswith semi-circular forms made out of soil and/or stones found

Fig. 1. Micro-basins built of earth and stone walls around olive trees.

M. Previati et al. / Agricultural Water Management 97 (2010) 449–456450

within and on the hillside, around every cultivated tree. Thesemicro-basins are comparable to little barriers built on the slopewith the aim to retain water in situ (and eroded soil) or to slowdown the runoff water velocity (Fig. 1). In the area considered forthis study, the farmers are aware of the economical gain generatedby this kind of management (Mancuso and Castellani, 2005).However, so far little hydrological evaluations of this type ofapplications exist in the literature (Boers and Ben-Asher, 1982;Sanchez-Cohen et al., 1997; Sepaskhah and Fooladmand, 2004;Schiettecatte et al., 2005).

The objective of this study is to quantify the soil water storageefficiency of micro-basins in relation to soil type, farm manage-ment and soil depth. To do so, water content measurements carriedout by time domain reflectometry (TDR) method are taken insideand outside the micro-basins. The results obtained with andwithout soil water harvesting are compared.

Qualitative assessment of infiltration capacity will also beperformed.

Suggestions for enhancing good management practices ofmicro-basins performances are presented.

2. Materials and methods

2.1. Experimental site

The highlands of Central Tunisia are known as ‘‘Tell’’; they arean extension of Algerian ‘‘Tell’’. The climate can be considered assemi-arid to arid. For this study, 15 farms with micro-basins areselected, all located in the Kairouan district. This area ischaracterized by an average annual rainfall of 300 mm, whichgradually decreases from north to south down to 240 mm (whenaveraged over the last 35 years).

Table 1Micro-basin locations and relative morphologic data.

Farm no. Soil type Latitude Longitude Slope (%)

1 Cambisols 568,055 3,949,631 33

2 Cambisols 571,371 3,953,207 27

3 Kastanozems 577,001 3,948,214 14

4 Arenosols 568,342 3,942,826 21

5 Calcisols 55,2852 3,911,191 16

6 Calcisols 568,622 3,951,355 26

7 Cambisols 568,567 3,942,923 9

8 Kastanozems 576,781 3,947,481 19

9 Calcisols 553,004 3,911,052 21

11 Arenosols 567,537 3,941,074 17

12 Calcisols 555,357 3908596 18

13 Kastanozems 576,715 3,947,598 10

14 Calcisols 555,831 3,914,163 12

15 Calcisols 555,561 391,4424 5

16 Calcisols 552,825 3,911,421 15

The selected farm population consists of 9 farms with one-year-old micro-basins and 6 farms with five-year-old micro-basins. Thewalls of all micro-basins are built from soil and stones and thevegetation concerns olive trees.

The one-year-old micro-basins are located on Cambisol,Kastanozem, and Arenosol soils; the profiles are characterized asfollows: (i) the Cambisols with a very fine textured cambic horizon;(ii) the Kastanozems with a mollic horizon with a moist chromavalue of more than 2 to a depth of at least 20 cm (FAO, 1998); and(iii) the Arenosols composed of a sandy texture to a depth of at least100 cm from the soil surface.

The five-year-old micro-basins are located in the Hajeb el Ayoun

area on Calcisol soils (FAO, 1998), characterized by a calcic orpetrocalcic horizon within 100 cm of the surface.

For each micro-basin and its associated micro-watershed,measurements are carried out to evaluate vegetation, soil tillageand slope, and surface characteristics (Table 1).

The geographical position of the study area is given in Fig. 2; theUTM coordinates of each farm are located between 55,2825–57,7001 latitude and 3,908,596–3,953,207 longitude (Table 1)with an altitude range between 130 and 350 m a.s.l.

2.2. Measurements

This paper contains a preliminary assessment of the value ofsoil hydraulic data for the management of olive trees in a hillysemi-arid area.

All collected measurements are presented, but the discussionmainly regards the water content values, therefore the infiltrationdata has only been analyzed in a qualitative way in this paper.However, information about texture and structure collected byevaluating the soil hydrological behaviour through falling headtests, can lead, by knowing the initial water content measurementsconditions, to estimate the full set of retention and hydraulicconductivity curves of the considered soils and use physicallybased simulation models (e.g., Lassabatere et al., 2006).

2.2.1. Soil parameters

Two pits of 1 m depth (one inside and one outside the soil andstone wall) are dug for every micro-basin to examine thepedological profiles. Soil texture, soil structure, skeleton, colour,and organic matter are analyzed at three different depths. Theresults are shown in Table 2.

2.2.2. Soil water content

Measurements are carried out by an ensemble of a three-wireprobe connected to a Tektronix 1502C TDR cable tester using a50 V coaxial cable and a BNC connection. The Roth et al. (1990)

Vegetation

cover (%)

Tillage Catchment

area (m2)

Micro-basin

area (m2)

20 No 70 18.4

15 Outside (old) 36 12.5

5 No 51 29.3

2 No 33 29.0

5 Inside (superficial) 63 32.3

10 Inside and outside 76 28.1

2 No 346 27.3

20 No 51 10.8

10 Inside 35 22.8

1 No 79 29.0

5 Outside (recent) 35 13.8

20 Inside 48 18.5

10 Inside 350 7.0

10 No 40 7.3

10 No 410 28.5

Fig. 2. The Kairouan district in Tunisia (North Africa). The enumerated micro-basins are in the rural area of Haffouz, Hajeb el Ayoun, and Kairouan; they are part of a FAO

project for soil and water conservation (FAO, 2004).

Table 2Texture profiles and organic matter analyzed for every micro-basin at three different depths. Data are presented following the FAO (1998) classification system.

Soil type

Cambisols Kastanozems Arenosols Calcisols

Farm no. 1 2 7 3 8 13 4 11 5 6 9 12 14 15 16

Soil depth (cm)

0–15 Sand % 39 31 83 27 51 63 87 93 71 40 73 75 54 57 77

Silt % 37 28 5 53 28 20 5 5 18 35 16 13 28 31 21

Clay % 24 41 12 20 21 17 8 2 11 25 11 12 18 12 2

O.M. % 1.8 0.9 0.0 2.4 2.5 0.6 1.1 0.0 1.3 1.1 0.0 0.0 0.7 1.7 0.8

30 Sand % 39 31 37 27 51 63 95 97 38 40 76 75 61 73 77

Silt % 37 28 13 53 28 20 3 2 29 35 13 13 27 20 21

Clay % 24 41 50 20 21 17 2 1 33 25 11 12 12 7 2

O.M. % 1.8 0.9 0.2 2.4 2.5 0.6 0.3 0.0 0.8 1.1 0.2 0.0 0.5 0.0 0.8

60 Sand % 27 13 37 97 96 36 39 76 75 69 73 67

Silt % 47 35 13 BED BED BED 1 2 39 34 13 15 19 20 29

Clay % 26 52 50 ROCK ROCK ROCK 2 2 25 27 11 10 12 7 4

O.M. % 1.3 0.9 0.0 0.2 0.1 0.7 0.5 0.2 0.1 0.1 0.0 0.0

M. Previati et al. / Agricultural Water Management 97 (2010) 449–456 451

relation is used for calculating water content from permittivity.The probes are made of two stainless steel rods (15 cm in length) of0.5 cm diameter spaced by a nylon spacer at a distance of 5 cm; thisto respect the 1/10 ratio between diameter and distance assuggested by Knight (1992).

For every soil profile, soil water content measurements aretaken inside and outside the 15 micro-basins by the use of a set oftwo-rod TDR probes inserted into the soil at the depths: 0–15 cmvertically, and 30 and 60 cm horizontally. The survey is conductedover two different time periods: spring 2003 (February–April) andautumn 2003 (October–December).

2.2.3. Infiltration tests

Falling head single ring infiltrometer tests (e.g., Lassabatereet al., 2006; Touma et al., 2007) are carried out by the use of aPVC cylinder (11 cm in diameter) slightly inserted into the soiland filled with 10 cm of water height. The infiltration time isrecorded every centimeter of falling water head until all water isinfiltrated.

Infiltration tests are carried out inside and outside the 15 micro-basins giving a total amount of 30 measurements.

3. Results and discussion

3.1. Soil water content

The soil water content measurements are analyzed as afunction of soil type, farm management and soil depth.

3.1.1. Soil type

Four soil types are compared, i.e., Cambisols, Kastanozems,Arenosols and Calcisols. The first three soil types are encountered atthe one-year-old micro-basins, whereas the Calcisols are found forthe five-year-old micro-basins. Water content values are measuredsimultaneously inside and outside the micro-basins. The resultsare presented in Fig. 3. Two time periods are chosen for themeasurements: spring time and autumn.

The overall comparison between the inside and outsidemeasurements shows that the water content values measuredinside the micro-basins are generally higher than those monitoredoutside the micro-basins. However, and in spite of this generaltendency, some noticeable effects due to soil type and time periodare observed.

Fig. 3. Comparison between volumetric water content measurements taken inside and outside the micro-basins. The results are plotted as a function of time period of the year

(i.e., spring and autumn), and soil type, i.e., Cambisols (a), Kastanozems (b), Arenosols (c), and Calcisols (d).

Table 3Average water content values ðuÞ calculated for respectively the autumn and spring data series measured inside and outside of the micro-basin for all four soil types. The

suffices ‘‘Ins’’, ‘‘Out’’, ‘‘Spr’’ and ‘‘Aut’’ refer to respectively the inside, the outside, the spring and the autumn data series.

Soil type Location Average values Spring Autumn General

Spr. Aut. Overall Ins. Out. Ins. Out. Ins. Out.

Cambisols Farm Ins. 0.271 0.377 0.324

1 Out. 0.228 0.308 0.268

Farm Ins. 0.352 0.460 0.406 0.306 0.227 0.425 0.295 0.366 0.261

2 Out. 0.194 0.253 0.224

Farm Ins. 0.294 0.439 0.367

7 Out. 0.258 0.325 0.291

Kastanozems Farm Ins. 0.197 0.161 0.179

3 Out. 0.176 0.192 0.184

Farm Ins. 0.068 0.122 0.095

8 Out. 0.061 0.142 0.101 0.163 0.120 0.156 0.197 0.160 0.158

Farm Ins. 0.224 0.186 0.205

13 Out. 0.122 0.256 0.189

Arenosols Farm Ins. 0.068 0.177 0.123

4 Out. 0.041 0.152 0.096

Farm Ins. 0.080 0.154 0.117 0.074 0.056 0.165 0.134 0.120 0.095

11 Out. 0.071 0.116 0.094

Calcisols Farm Ins. 0.173 0.138 0.155

5 Out. 0.134 0.196 0.165

Farm Ins. 0.088 0.250 0.169

6 Out. 0.068 0.207 0.138

Farm Ins. 0.072 0.170 0.121

9 Out. 0.061 0.125

Farm Ins. 0.200 0.076 0.138 0.158 0.155 0.118 0.162 0.138 0.158

12 Out. 0.113 0.182 0.148 0.147a 0.117a 0.175a 0.151a 0.161a 0.134a

Farm Ins. 0.195 0.223 0.209

14 Out. 0.117 0.185 0.151

Farm 0.197 0.100 0.149

15 0.186 0.104 0.145

Farm 0.183 0.129 0.156

16 0.151 0.132 0.141

a Without considering the farms 5 and 12 (which present incorrect labour practices) the average values show the proper micro-basin functions.

M. Previati et al. / Agricultural Water Management 97 (2010) 449–456452

Fig. 4. Surface water storage action on a Cambisol micro-basin during a rainy day

(February 28, 2003).

M. Previati et al. / Agricultural Water Management 97 (2010) 449–456 453

The results obtained from the Cambisols (Fig. 3a) are the mostinteresting. At some points, an increase in water stock of 30% ismeasured. As these soils are characterized by the presence of finematerials over great depths, they exhibit a good water retentioncapacity combined with high water storage. It explains why thevalues of the volumetric water content are bigger than thoseobserved for the other soil types. Calculating the average watercontent value (u) for respectively the autumn and spring dataseries of both the inside and outside measurements (Table 3), theCambisol u-values are systematically the biggest. Denoting the‘‘inside autumn’’ measurements with the suffix ‘‘ins-aut’’, uins-aut ¼0:425 cm3=cm3 for the Cambisols against uins-aut ¼ 0:156 cm3=cm3

for the Kastanozem soils, uins-aut ¼ 0:165 cm3=cm3 for the Arenosols,and uins-aut ¼ 0:175 cm3=cm3 for the Calcisols.

The differences between inside and outside measurements areparticularly visible during the autumn period when rains are moreabundant. During spring time the measurements taken outside themicro-basins benefit of some rare rainfall events. This explainswhy the difference between uins-aut and uout-aut is more importantthan that observed for uins-spr and uout-spr (where the suffix ‘‘spr’’refers to the spring period). For illustration, Fig. 4 shows thesurface storage on a Cambisol micro-basin after a rainfall event.

The Kastanozems (Fig. 3b) are loamy textured fertile soils with ahigh quantity of organic matter. Unfortunately, the Kastanozem soilspresented in this study form a very thin soil layer. Consequently, thewater storage capacity is strongly reduced. For these soil profiles, theuse of micro-basins is not advisable as no strong benefit can beexpected. This is clearly demonstrated by the average values of

Fig. 5. Comparison between correct tillage practices (a) observed for farm

uins-gen and uout-gen which are quasi identical, e.g., uins-aut ¼0:160 cm3=cm3 and uout-aut ¼ 0:158 cm3=cm3. When comparingthe two time periods, the autumn data show a big scatter which isdifficult to explain a priori. The spring data are more consistent.

The results obtained for the Arenosols (Fig. 3c) are similar tothose observed for the Cambisols. The autumn data show a muchwetter soil profile inside than outside the micro-basins withuins-aut ¼ 0:165 cm3=cm3 and uout-aut ¼ 0:134 cm3=cm3. Accordingto the U.S. Department of Agriculture (Soil Survey Laboratory Staff,1992) the particle-size distribution of Arenosols is mainlycomposed of sand particles with a diameter between 0.02 and2 mm (7th USDA Soil Classification system). Consequently, thewater holding capacity of the Arenosols is strongly reduced and thenumerical value uins-aut ¼ 0:165 cm3=cm3 is small as comparedwith that of the Cambisols uins-aut ¼ 0:425 cm3=cm3.

Once again (except Kastanozems, as explained before), thedifferences between inside and outside measurements areparticularly visible for the autumn period when rains are scarce.

The last class of soils concerns the Calcisols (Fig. 3d). For bothspring and autumn time, the inside measurements are wetter thanthose observed outside the micro-basin, e.g., uins-aut ¼0:175 cm3=cm3 and uout-aut ¼ 0:151 cm3=cm3. However, as willbe shown below, the results obtained for this soil type areparticularly sensitive to the farm management such as tillagepractices.

Resuming, the above results show that the use of micro-basinshas a positive effect on the water holding capacity of the soil profileespecially for fine textured soil types such as the Cambisols. Thetime period also has an influence with the micro-basins operatingmore efficiently during the less frequent rainfall periods ofautumn.

3.1.2. Farm management

As to the influence of farm management, the effect of tillage isanalyzed next. The five-year-old micro-basins are chosen for thesetests, because they are all built on the same type of soil (Calcisols).The results of two farms (no. 6 and no. 12) are selected forcomparison. Generally speaking, tillage practices such as plough-ing, or scarifying, enhance the fine textured soil profiles and avoidthe progressive formation of calcipans (hard impermeable crustlayers) over time. Hence, when tillage practices are carried outcorrectly, farm management has definitely a positive effect on thewater storage capacity of soils (Fig. 5a, farm no. 6). However, whenthe tillage practices are carried out incorrectly the results arerapidly deteriorating. The example is shown in Fig. 5b, observed forfarm no. 12. During the spring period, the micro-basin functions

no. 6 and incorrect farm management measured for farm no. 12 (b).

M. Previati et al. / Agricultural Water Management 97 (2010) 449–456454

perfectly enhancing soil water storage in the soil profile. Due tosuccessive wetting and drying, crust forming takes place at the soilsurface sealing off slowly but surely the soil profile. This effect isclearly put in evidence by the autumn measurement. While theoutside water content measurements react normally to rainfall (nocrust forming has taken place outside the micro-basins), the soilprofile inside the micro-basin is drying out due to the fact thatinfiltration is blocked off by the crust layer at the soil surface.

The above results are extremely illustrative. They show theimportance of accompanying micro-basin practices with appro-priate farm management. Since water clogging within the micro-basins have the tendency to generate crust formation for most soiltypes (and not only for the Calcisols of the above example), themaintenance of the soil surface is one of the prime conditionsnecessary for successful water harvesting by the use of micro-basinpractices.

Associated with the problem of crust formation, there is aproblem of soil erosion and soil conservation. During harvesting ofrainwater, small soil particles are transported over the slopesurface to the micro-basin. This erosion process generally concernssmall particles with a diameter smaller than 0.2 mm. Whenestimating the erosion losses for the different farms by the use ofthe USLE-method (Wischmeier and Smith, 1978), a volume of 2–100 t ha�1 y�1 is calculated. This volume is partially dischargedwithin the micro-basin. An example is given in Fig. 6. Once again,tillage inside the micro-basins is the recommended farm practiceto overcome the problem of soil erosion in relation with its positiveeffects on water infiltration enhancements which leads to thedeposition of soil particles eroded on the slopes.

3.1.3. Soil depth

The last test concerns the effect of soil depth. For this test, theensemble of water content measurements is classified in threegroups as a function of soil depth. The first group of data concernsthe top layer with z < 15 cm; the second data set refers to the layerz = 30 cm; and the last group applies to z = 60 cm. The results areshown in Fig. 7a–c.

The soil water storage efficiency increases with depth. Waterstorage at deeper depths is important as it allows water storageover longer periods. This is particularly efficient when evaporationlosses are avoided.

3.2. Infiltration behaviour

When carrying out infiltration experiments inside and outsidethe micro-basin, different infiltration dynamics are observed as afunction of soil type. The time required to infiltrate 10 cm of wateris plotted in Fig. 8.

Fig. 6. The effects of surface erosion into a sandy soil micro-basin.

Fig. 7. Comparison between water content measurements taken inside and outside

the micro-basins plotted as a function of soil depth. (a) Concerns the top layer with

z < 15 cm; (b) refers to the layer z = 30 cm; and (c) applies to z = 60 cm.

The comparison between the time intervals observed inside andoutside the micro-basins, shows for nearly all farms a longerinfiltration time measured inside the micro-basin. This delay iscertainly related to the fact that the water harvesting trails finematerial in suspension. Once discharged within the micro-basin, itslightly modifies the textural composition of the surface layer. The

Fig. 8. Infiltration time necessary to infiltrate 10 cm of water head measured inside

and outside the micro-basins of every single farm.

M. Previati et al. / Agricultural Water Management 97 (2010) 449–456 455

finer the soil material, the slower the infiltration dynamics andhence the longer the time period to infiltrate 10 cm of water head.

The Arenosols show fast infiltration behaviour. This is explainedby the fact that the sandy soils are characterized by relatively highpermeability in the absence of fine material. As the Kastanozem

layer is thin, the infiltration dynamics observed for this soil typerather reflect the infiltration behaviour of the underlying sandmatrix.

4. Conclusions

In this study micro-basin slope management for rainfallharvesting is evaluated. To do so, water content measurementsinside and outside the micro-basins are compared for different soiltypes.

The overall comparison between the inside and outsidemeasurements shows that the water content values measuredinside the micro-basins are generally higher than those monitoredoutside the micro-basins. However, and in spite of this generaltendency, some noticeable effects due to soil type, time of the year,farm management and soil depth are observed.

As to soil type, the above results show that the use of micro-basins have an increasingly important effect for fine textured soils.When comparing the yearly averaged differences of the insideminus outside water content data taken in the soil surface layer(z � 15 cm), an increase of 0.10 cm3/cm3 is observed for finetextured soils (i.e., Cambisols) against only 0.03 cm3/cm3 for sandysoils (Arenosols).

The time period of the year also has an influence. The lessfrequent the rain is (e.g., the autumn period) the more efficientlyoperate the micro-basins.

But among all, farm management is shown to be one of the mostimportant key factors to succeed water harvesting by the use ofmicro-basins. Tillage practices not only avoid crust forming at thesoil surface, but also enhance dry-mulching effects which maybecome increasingly effectual for deeper layers of the soil profile.

Obviously, the above list of key factors affecting the efficiency ofwater harvesting by the use of micro-basins should not be seen asexhaustive. Other effects, such as rainfall intensity and duration,catchment size, stones and rocky surfaces, also strongly influencethe efficiency of the micro-basins.

Acknowledgements

This work has been possible thanks to the FAO Project ‘‘FAO

GCP-TUN-028-ITA’’. It was partially funded by the above-men-tioned FAO Project and the Italian Ministry of Research throughProject PRIN 2007: ‘‘Experimental measurements of the atmo-sphere–vegetation–soil interaction processes and their response toclimate change’’.

Special thanks go out to the Tunisian Headquarters, their wholestaff, and the Kairouan displace unit. Our gratitude also goes to thereviewers for their timely and accurate reviews and for their helpin improving this work.

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