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Evaluation of soil water storage efficiency for rainfall harvesting on hillslope micro-basins built using time domain reflectometry measurements
Agricultural Water Management 97 (2010) 449–456
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Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat
Evaluation of soil water storage efficiency for rainfall harvesting on hillslope micro-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
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 July 2009 Accepted 8 November 2009
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. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Water harvesting Water conservation TDR Semi-arid areas Micro-basins
1. Introduction The hydrological cycle in soil begins with the entry of water by infiltration, then continues with a temporary storage of water in the soil, and ends with its removal from the soil by drainage, evaporation, and plant uptake. Infiltration determines how much water enters the root zone and how much will runoff. Runoff is considered as the portion of water that exceeds the infiltration capacity. Hence, the rate of infiltration affects not only the water economy of vegetation, but also the potential amount of surface runoff and the risk of soil erosion in semi-arid zone cultivated hillslopes (Senay and Verdin, 2004; Schiettecatte et al., 2005; Oweis and Hachum, 2006). Water harvesting systems for runoff water collection and storage represent an attractive solution for resolving water scarcity in various parts of the world (e.g., Tabor, 1995; Van Wesemael et al., 1998; Oweis and Hachum, 2006; Schiettecatte et al., 2005; Frot et al., 2008). According to the Food and Agriculture 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: [email protected] (M. Previati). 0378-3774/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2009.11.004
characterized by irregular distribution over space and time of the water resources. Historically speaking, agriculture using surface runoff and rain harvesting techniques was extensively practiced as early as thousands of years ago. As recently reviewed by Vohland and Barry (2009), in these arid and semi-arid zones, a huge variety of traditional as well as innovative in situ rainwater harvesting practices exist. Typical in situ structures used on slope management are linear structures (e.g., embankment of stone and/or earth, or grass strips) which can be sophisticated, such as the teras system in Sudan (van Dijk, 1997) and terracing as practiced in East Africa. Semi-circular bunds are common in the arid and semi-arid zones of Northern Africa and Sahel (e.g., micro-basins or half moons or demi lunes (Barry and Sonou, 2003)). Pitting cultivation is practiced in different 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 Matengo people in East Africa (Critchley and Mutunga, 2003; Kato, 2001; Mati and Lange, 2003; Malley et al., 2004). These various systems differ mainly in the size of the pits. In general, biomass production is improved by applying a layer of mulch in the pit before planting. Other approaches, such as conservation tillage, improve infiltration ability on the field scale (Stroosnijder, 2003). Micro-basin watershed management is realized by little walls with semi-circular forms made out of soil and/or stones found
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Fig. 1. Micro-basins built of earth and stone walls around olive trees.
within and on the hillside, around every cultivated tree. These micro-basins are comparable to little barriers built on the slope with the aim to retain water in situ (and eroded soil) or to slow down the runoff water velocity (Fig. 1). In the area considered for this study, the farmers are aware of the economical gain generated by this kind of management (Mancuso and Castellani, 2005). However, so far little hydrological evaluations of this type of applications 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 storage efficiency of micro-basins in relation to soil type, farm management and soil depth. To do so, water content measurements carried out by time domain reflectometry (TDR) method are taken inside and outside the micro-basins. The results obtained with and without soil water harvesting are compared. Qualitative assessment of infiltration capacity will also be performed. Suggestions for enhancing good management practices of micro-basins performances are presented. 2. Materials and methods 2.1. Experimental site The highlands of Central Tunisia are known as ‘‘Tell’’; they are an extension of Algerian ‘‘Tell’’. The climate can be considered as semi-arid to arid. For this study, 15 farms with micro-basins are selected, all located in the Kairouan district. This area is characterized by an average annual rainfall of 300 mm, which gradually decreases from north to south down to 240 mm (when averaged over the last 35 years).
The selected farm population consists of 9 farms with one-yearold micro-basins and 6 farms with five-year-old micro-basins. The walls of all micro-basins are built from soil and stones and the vegetation concerns olive trees. The one-year-old micro-basins are located on Cambisol, Kastanozem, and Arenosol soils; the profiles are characterized as follows: (i) the Cambisols with a very fine textured cambic horizon; (ii) the Kastanozems with a mollic horizon with a moist chroma value 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 least 100 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 or petrocalcic horizon within 100 cm of the surface. For each micro-basin and its associated micro-watershed, measurements are carried out to evaluate vegetation, soil tillage and slope, and surface characteristics (Table 1). The geographical position of the study area is given in Fig. 2; the UTM 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 of soil hydraulic data for the management of olive trees in a hilly semi-arid area. All collected measurements are presented, but the discussion mainly regards the water content values, therefore the infiltration data has only been analyzed in a qualitative way in this paper. However, information about texture and structure collected by evaluating the soil hydrological behaviour through falling head tests, can lead, by knowing the initial water content measurements conditions, to estimate the full set of retention and hydraulic conductivity curves of the considered soils and use physically based simulation models (e.g., Lassabate`re et al., 2006). 2.2.1. Soil parameters Two pits of 1 m depth (one inside and one outside the soil and stone wall) are dug for every micro-basin to examine the pedological profiles. Soil texture, soil structure, skeleton, colour, and organic matter are analyzed at three different depths. The results are shown in Table 2. 2.2.2. Soil water content Measurements are carried out by an ensemble of a three-wire probe connected to a Tektronix 1502C TDR cable tester using a 50 V coaxial cable and a BNC connection. The Roth et al. (1990)
Table 1 Micro-basin locations and relative morphologic data. Farm no.
Soil type
Latitude
Longitude
Slope (%)
Vegetation cover (%)
Tillage
Catchment area (m2)
Micro-basin area (m2)
1 2 3 4 5 6 7 8 9 11 12 13 14 15 16
Cambisols Cambisols Kastanozems Arenosols Calcisols Calcisols Cambisols Kastanozems Calcisols Arenosols Calcisols Kastanozems Calcisols Calcisols Calcisols
568,055 571,371 577,001 568,342 55,2852 568,622 568,567 576,781 553,004 567,537 555,357 576,715 555,831 555,561 552,825
3,949,631 3,953,207 3,948,214 3,942,826 3,911,191 3,951,355 3,942,923 3,947,481 3,911,052 3,941,074 3908596 3,947,598 3,914,163 391,4424 3,911,421
33 27 14 21 16 26 9 19 21 17 18 10 12 5 15
20 15 5 2 5 10 2 20 10 1 5 20 10 10 10
No Outside (old) No No Inside (superficial) Inside and outside No No Inside No Outside (recent) Inside Inside No No
70 36 51 33 63 76 346 51 35 79 35 48 350 40 410
18.4 12.5 29.3 29.0 32.3 28.1 27.3 10.8 22.8 29.0 13.8 18.5 7.0 7.3 28.5
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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 2 Texture 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 Farm no. Soil depth (cm) 0–15 Sand % Silt % Clay % O.M. %
Kastanozems
Arenosols
Calcisols
1
2
7
3
8
13
4
11
5
6
9
12
14
15
16
39 37 24 1.8
31 28 41 0.9
83 5 12 0.0
27 53 20 2.4
51 28 21 2.5
63 20 17 0.6
87 5 8 1.1
93 5 2 0.0
71 18 11 1.3
40 35 25 1.1
73 16 11 0.0
75 13 12 0.0
54 28 18 0.7
57 31 12 1.7
77 21 2 0.8
30
Sand % Silt % Clay % O.M. %
39 37 24 1.8
31 28 41 0.9
37 13 50 0.2
27 53 20 2.4
51 28 21 2.5
63 20 17 0.6
95 3 2 0.3
97 2 1 0.0
38 29 33 0.8
40 35 25 1.1
76 13 11 0.2
75 13 12 0.0
61 27 12 0.5
73 20 7 0.0
77 21 2 0.8
60
Sand % Silt % Clay % O.M. %
27 47 26 1.3
13 35 52 0.9
37 13 50 0.0
BED ROCK
BED ROCK
BED ROCK
97 1 2 0.2
96 2 2 0.1
36 39 25 0.7
39 34 27 0.5
76 13 11 0.2
75 15 10 0.1
69 19 12 0.1
73 20 7 0.0
67 29 4 0.0
relation is used for calculating water content from permittivity. The probes are made of two stainless steel rods (15 cm in length) of 0.5 cm diameter spaced by a nylon spacer at a distance of 5 cm; this to respect the 1/10 ratio between diameter and distance as suggested by Knight (1992). For every soil profile, soil water content measurements are taken inside and outside the 15 micro-basins by the use of a set of two-rod TDR probes inserted into the soil at the depths: 0–15 cm vertically, and 30 and 60 cm horizontally. The survey is conducted over two different time periods: spring 2003 (February–April) and autumn 2003 (October–December). 2.2.3. Infiltration tests Falling head single ring infiltrometer tests (e.g., Lassabate`re et al., 2006; Touma et al., 2007) are carried out by the use of a PVC cylinder (11 cm in diameter) slightly inserted into the soil and filled with 10 cm of water height. The infiltration time is recorded every centimeter of falling water head until all water is infiltrated. Infiltration tests are carried out inside and outside the 15 microbasins giving a total amount of 30 measurements.
3. Results and discussion 3.1. Soil water content The soil water content measurements are analyzed as a function 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 at the one-year-old micro-basins, whereas the Calcisols are found for the five-year-old micro-basins. Water content values are measured simultaneously inside and outside the micro-basins. The results are presented in Fig. 3. Two time periods are chosen for the measurements: spring time and autumn. The overall comparison between the inside and outside measurements shows that the water content values measured inside the micro-basins are generally higher than those monitored outside the micro-basins. However, and in spite of this general tendency, some noticeable effects due to soil type and time period are observed.
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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 3 Average water content values ðu¯ Þ 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
Cambisols
Kastanozems
Arenosols
Calcisols
a
Location
Average values
Spring
Spr.
Aut.
Overall
Farm 1 Farm 2 Farm 7
Ins. Out. Ins. Out. Ins. Out.
0.271 0.228 0.352 0.194 0.294 0.258
0.377 0.308 0.460 0.253 0.439 0.325
0.324 0.268 0.406 0.224 0.367 0.291
Farm 3 Farm 8 Farm 13
Ins. Out. Ins. Out. Ins. Out.
0.197 0.176 0.068 0.061 0.224 0.122
0.161 0.192 0.122 0.142 0.186 0.256
0.179 0.184 0.095 0.101 0.205 0.189
Farm 4 Farm 11 Farm 5 Farm 6 Farm 9 Farm 12 Farm 14 Farm 15 Farm 16
Ins. Out. Ins. Out. Ins. Out. Ins. Out. Ins. Out. Ins. Out. Ins. Out.
0.068 0.041 0.080 0.071 0.173 0.134 0.088 0.068 0.072 0.061 0.200 0.113 0.195 0.117 0.197 0.186 0.183 0.151
0.177 0.152 0.154 0.116 0.138 0.196 0.250 0.207 0.170 0.125 0.076 0.182 0.223 0.185 0.100 0.104 0.129 0.132
0.123 0.096 0.117 0.094 0.155 0.165 0.169 0.138 0.121 0.138 0.148 0.209 0.151 0.149 0.145 0.156 0.141
Autumn
General
Ins.
Out.
Ins.
Out.
Ins.
Out.
0.306
0.227
0.425
0.295
0.366
0.261
0.163
0.120
0.156
0.197
0.160
0.158
0.074
0.056
0.165
0.134
0.120
0.095
0.158 0.147a
0.155 0.117a
0.118 0.175a
0.162 0.151a
0.138 0.161a
0.158 0.134a
Without considering the farms 5 and 12 (which present incorrect labour practices) the average values show the proper micro-basin functions.
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Fig. 4. Surface water storage action on a Cambisol micro-basin during a rainy day (February 28, 2003).
The results obtained from the Cambisols (Fig. 3a) are the most interesting. At some points, an increase in water stock of 30% is measured. As these soils are characterized by the presence of fine materials over great depths, they exhibit a good water retention capacity combined with high water storage. It explains why the values of the volumetric water content are bigger than those observed for the other soil types. Calculating the average water content value (u¯ ) for respectively the autumn and spring data series of both the inside and outside measurements (Table 3), the Cambisol u¯ -values are systematically the biggest. Denoting the ‘‘inside autumn’’ measurements with the suffix ‘‘ins-aut’’, u¯ ins-aut ¼ 0:425 cm3 =cm3 for the Cambisols against u¯ ins-aut ¼ 0:156 cm3 =cm3 for the Kastanozem soils, u¯ ins-aut ¼ 0:165 cm3 =cm3 for the Arenosols, and u¯ ins-aut ¼ 0:175 cm3 =cm3 for the Calcisols. The differences between inside and outside measurements are particularly visible during the autumn period when rains are more abundant. During spring time the measurements taken outside the micro-basins benefit of some rare rainfall events. This explains why the difference between u¯ ins-aut and u¯ out-aut is more important than that observed for u¯ ins-spr and u¯ out-spr (where the suffix ‘‘spr’’ refers to the spring period). For illustration, Fig. 4 shows the surface storage on a Cambisol micro-basin after a rainfall event. The Kastanozems (Fig. 3b) are loamy textured fertile soils with a high quantity of organic matter. Unfortunately, the Kastanozem soils presented in this study form a very thin soil layer. Consequently, the water storage capacity is strongly reduced. For these soil profiles, the use of micro-basins is not advisable as no strong benefit can be expected. This is clearly demonstrated by the average values of
453
u¯ ins-gen and u¯ out-gen which are quasi identical, e.g., u¯ ins-aut ¼ 0:160 cm3 =cm3 and u¯ out-aut ¼ 0:158 cm3 =cm3 . When comparing the two time periods, the autumn data show a big scatter which is difficult to explain a priori. The spring data are more consistent. The results obtained for the Arenosols (Fig. 3c) are similar to those observed for the Cambisols. The autumn data show a much wetter soil profile inside than outside the micro-basins with u¯ ins-aut ¼ 0:165 cm3 =cm3 and u¯ out-aut ¼ 0:134 cm3 =cm3 . According to the U.S. Department of Agriculture (Soil Survey Laboratory Staff, 1992) the particle-size distribution of Arenosols is mainly composed of sand particles with a diameter between 0.02 and 2 mm (7th USDA Soil Classification system). Consequently, the water holding capacity of the Arenosols is strongly reduced and the numerical value u¯ ins-aut ¼ 0:165 cm3 =cm3 is small as compared with that of the Cambisols u¯ ins-aut ¼ 0:425 cm3 =cm3 . Once again (except Kastanozems, as explained before), the differences between inside and outside measurements are particularly visible for the autumn period when rains are scarce. The last class of soils concerns the Calcisols (Fig. 3d). For both spring and autumn time, the inside measurements are wetter than those observed outside the micro-basin, e.g., u¯ ins-aut ¼ 0:175 cm3 =cm3 and u¯ out-aut ¼ 0:151 cm3 =cm3 . However, as will be shown below, the results obtained for this soil type are particularly sensitive to the farm management such as tillage practices. Resuming, the above results show that the use of micro-basins has a positive effect on the water holding capacity of the soil profile especially for fine textured soil types such as the Cambisols. The time period also has an influence with the micro-basins operating more efficiently during the less frequent rainfall periods of autumn. 3.1.2. Farm management As to the influence of farm management, the effect of tillage is analyzed next. The five-year-old micro-basins are chosen for these tests, because they are all built on the same type of soil (Calcisols). The results of two farms (no. 6 and no. 12) are selected for comparison. Generally speaking, tillage practices such as ploughing, or scarifying, enhance the fine textured soil profiles and avoid the progressive formation of calcipans (hard impermeable crust layers) over time. Hence, when tillage practices are carried out correctly, farm management has definitely a positive effect on the water storage capacity of soils (Fig. 5a, farm no. 6). However, when the tillage practices are carried out incorrectly the results are rapidly deteriorating. The example is shown in Fig. 5b, observed for farm no. 12. During the spring period, the micro-basin functions
Fig. 5. Comparison between correct tillage practices (a) observed for farm no. 6 and incorrect farm management measured for farm no. 12 (b).
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perfectly enhancing soil water storage in the soil profile. Due to successive wetting and drying, crust forming takes place at the soil surface sealing off slowly but surely the soil profile. This effect is clearly put in evidence by the autumn measurement. While the outside water content measurements react normally to rainfall (no crust forming has taken place outside the micro-basins), the soil profile inside the micro-basin is drying out due to the fact that infiltration is blocked off by the crust layer at the soil surface. The above results are extremely illustrative. They show the importance of accompanying micro-basin practices with appropriate farm management. Since water clogging within the microbasins have the tendency to generate crust formation for most soil types (and not only for the Calcisols of the above example), the maintenance of the soil surface is one of the prime conditions necessary for successful water harvesting by the use of micro-basin practices. Associated with the problem of crust formation, there is a problem of soil erosion and soil conservation. During harvesting of rainwater, small soil particles are transported over the slope surface to the micro-basin. This erosion process generally concerns small particles with a diameter smaller than 0.2 mm. When estimating the erosion losses for the different farms by the use of the USLE-method (Wischmeier and Smith, 1978), a volume of 2– 100 t ha1 y1 is calculated. This volume is partially discharged within the micro-basin. An example is given in Fig. 6. Once again, tillage inside the micro-basins is the recommended farm practice to overcome the problem of soil erosion in relation with its positive effects on water infiltration enhancements which leads to the deposition of soil particles eroded on the slopes. 3.1.3. Soil depth The last test concerns the effect of soil depth. For this test, the ensemble of water content measurements is classified in three groups as a function of soil depth. The first group of data concerns the top layer with z < 15 cm; the second data set refers to the layer z = 30 cm; and the last group applies to z = 60 cm. The results are shown in Fig. 7a–c. The soil water storage efficiency increases with depth. Water storage at deeper depths is important as it allows water storage over longer periods. This is particularly efficient when evaporation losses are avoided. 3.2. Infiltration behaviour When carrying out infiltration experiments inside and outside the micro-basin, different infiltration dynamics are observed as a function of soil type. The time required to infiltrate 10 cm of water is plotted in Fig. 8.
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.
Fig. 6. The effects of surface erosion into a sandy soil micro-basin.
The comparison between the time intervals observed inside and outside the micro-basins, shows for nearly all farms a longer infiltration time measured inside the micro-basin. This delay is certainly related to the fact that the water harvesting trails fine material in suspension. Once discharged within the micro-basin, it slightly modifies the textural composition of the surface layer. The
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Special thanks go out to the Tunisian Headquarters, their whole staff, and the Kairouan displace unit. Our gratitude also goes to the reviewers for their timely and accurate reviews and for their help in improving this work. References
Fig. 8. Infiltration time necessary to infiltrate 10 cm of water head measured inside and outside the micro-basins of every single farm.
finer the soil material, the slower the infiltration dynamics and hence the longer the time period to infiltrate 10 cm of water head. The Arenosols show fast infiltration behaviour. This is explained by the fact that the sandy soils are characterized by relatively high permeability in the absence of fine material. As the Kastanozem layer is thin, the infiltration dynamics observed for this soil type rather reflect the infiltration behaviour of the underlying sand matrix. 4. Conclusions In this study micro-basin slope management for rainfall harvesting is evaluated. To do so, water content measurements inside and outside the micro-basins are compared for different soil types. The overall comparison between the inside and outside measurements shows that the water content values measured inside the micro-basins are generally higher than those monitored outside the micro-basins. However, and in spite of this general tendency, 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 microbasins have an increasingly important effect for fine textured soils. When comparing the yearly averaged differences of the inside minus outside water content data taken in the soil surface layer (z 15 cm), an increase of 0.10 cm3/cm3 is observed for fine textured soils (i.e., Cambisols) against only 0.03 cm3/cm3 for sandy soils (Arenosols). The time period of the year also has an influence. The less frequent the rain is (e.g., the autumn period) the more efficiently operate the micro-basins. But among all, farm management is shown to be one of the most important key factors to succeed water harvesting by the use of micro-basins. Tillage practices not only avoid crust forming at the soil surface, but also enhance dry-mulching effects which may become increasingly effectual for deeper layers of the soil profile. Obviously, the above list of key factors affecting the efficiency of water harvesting by the use of micro-basins should not be seen as exhaustive. Other effects, such as rainfall intensity and duration, catchment size, stones and rocky surfaces, also strongly influence the 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-mentioned FAO Project and the Italian Ministry of Research through Project PRIN 2007: ‘‘Experimental measurements of the atmosphere–vegetation–soil interaction processes and their response to climate change’’.
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Ngoro: an indigenous, sustainable and profitable soil, water and nutrient conservation system in Tanzania for sloping land. Soil Tillage Res. 77, 47–58. Mancuso, T., Castellani, L., 2005. Ame´nagement du Territoire pour la Conservation du Sol et Agriculture Pluviale: Re´sultats Economiques des Exploitations. D.E.I.A.F.A., Dep. Note. , 120 pp. Mati, B.M., Lange, M., 2003. Emerging practices in water management under rainfed agriculture in eastern Africa. In: Proceedings of the Symposium and Workshop on Water Conservation Technologies for Sustainable Dryland Agriculture in Sub-Saharan Africa (WCT) Bloem Spa Lodge and Conference Centre, Bloemfontein, South Africa, April 8–11. ARC-Institute for Soil, Climate and Water, Pretoria, South Africa, pp. 60–74. Ouedraogo, M., Kabore´, V., 1996. The ‘‘zai’’: a traditional technique for the rehabilitation of degraded land in the Yatenga, Burkina Faso. In: Reij, C., Scoones, I., Toulmin, C. (Eds.), Sustaining the Soil. Indigenous Soil and Water Conservation in Africa. pp. 80–92. Oweis, T., Hachum, A., 2006. Water harvesting and supplemental irrigation for improved water productivity of dry farming systems in West Asia and North Africa. Agric. Water Manage. 80, 57–73. Roth, K., Schulin, R., Fluhler, H., Attinger, W., 1990. Calibration of TDR for water content measurement using a composite dielectric approach. Water Resour. Res. 26, 2267–2273. Sanchez-Cohen, I., Lopes, V.L., Slack, D.C., Fogel, M.M., 1997. Water balance model for small-scale water harvesting system. J. Irrig. Drain. Eng. 123, 537–540. Schiettecatte, W., Ouessar, M., Gabriels, D., Tanghe, S., Heirman, S., Abdelli, F., 2005. Impact of water harvesting techniques on soil and water conservation: a case study on a micro catchment in Southeastern Tunisia. J. Arid Environ. 61, 297–313. Senay, G.B., Verdin, J.P., 2004. Developing index maps of water-harvest potential in Africa. Appl. Eng. Agric. 20, 789–799. Sepaskhah, A.R., Fooladmand, H.R., 2004. A computer model for design of microcatchment water harvesting system for rain-fed vineyard. Agric. Water Manage. 64, 213–232. Soil Survey Laboratory Staff, 1992. Soil Survey Laboratory Methods Manual. Soil Surv. Invest. Rep. 42 Version 2.0. USDA-SCS U.S. Gov. Print. Office, Washington, DC. Stroosnijder, L., 2003. Technologies for improving rain water use efficiency in semiarid Africa. In: Proceedings of the Symposium and Workshop on Water
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Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat
Evaluation of soil water storage efficiency for rainfall harvesting on hillslope micro-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
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 July 2009 Accepted 8 November 2009
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. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Water harvesting Water conservation TDR Semi-arid areas Micro-basins
1. Introduction The hydrological cycle in soil begins with the entry of water by infiltration, then continues with a temporary storage of water in the soil, and ends with its removal from the soil by drainage, evaporation, and plant uptake. Infiltration determines how much water enters the root zone and how much will runoff. Runoff is considered as the portion of water that exceeds the infiltration capacity. Hence, the rate of infiltration affects not only the water economy of vegetation, but also the potential amount of surface runoff and the risk of soil erosion in semi-arid zone cultivated hillslopes (Senay and Verdin, 2004; Schiettecatte et al., 2005; Oweis and Hachum, 2006). Water harvesting systems for runoff water collection and storage represent an attractive solution for resolving water scarcity in various parts of the world (e.g., Tabor, 1995; Van Wesemael et al., 1998; Oweis and Hachum, 2006; Schiettecatte et al., 2005; Frot et al., 2008). According to the Food and Agriculture 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: [email protected] (M. Previati). 0378-3774/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2009.11.004
characterized by irregular distribution over space and time of the water resources. Historically speaking, agriculture using surface runoff and rain harvesting techniques was extensively practiced as early as thousands of years ago. As recently reviewed by Vohland and Barry (2009), in these arid and semi-arid zones, a huge variety of traditional as well as innovative in situ rainwater harvesting practices exist. Typical in situ structures used on slope management are linear structures (e.g., embankment of stone and/or earth, or grass strips) which can be sophisticated, such as the teras system in Sudan (van Dijk, 1997) and terracing as practiced in East Africa. Semi-circular bunds are common in the arid and semi-arid zones of Northern Africa and Sahel (e.g., micro-basins or half moons or demi lunes (Barry and Sonou, 2003)). Pitting cultivation is practiced in different 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 Matengo people in East Africa (Critchley and Mutunga, 2003; Kato, 2001; Mati and Lange, 2003; Malley et al., 2004). These various systems differ mainly in the size of the pits. In general, biomass production is improved by applying a layer of mulch in the pit before planting. Other approaches, such as conservation tillage, improve infiltration ability on the field scale (Stroosnijder, 2003). Micro-basin watershed management is realized by little walls with semi-circular forms made out of soil and/or stones found
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Fig. 1. Micro-basins built of earth and stone walls around olive trees.
within and on the hillside, around every cultivated tree. These micro-basins are comparable to little barriers built on the slope with the aim to retain water in situ (and eroded soil) or to slow down the runoff water velocity (Fig. 1). In the area considered for this study, the farmers are aware of the economical gain generated by this kind of management (Mancuso and Castellani, 2005). However, so far little hydrological evaluations of this type of applications 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 storage efficiency of micro-basins in relation to soil type, farm management and soil depth. To do so, water content measurements carried out by time domain reflectometry (TDR) method are taken inside and outside the micro-basins. The results obtained with and without soil water harvesting are compared. Qualitative assessment of infiltration capacity will also be performed. Suggestions for enhancing good management practices of micro-basins performances are presented. 2. Materials and methods 2.1. Experimental site The highlands of Central Tunisia are known as ‘‘Tell’’; they are an extension of Algerian ‘‘Tell’’. The climate can be considered as semi-arid to arid. For this study, 15 farms with micro-basins are selected, all located in the Kairouan district. This area is characterized by an average annual rainfall of 300 mm, which gradually decreases from north to south down to 240 mm (when averaged over the last 35 years).
The selected farm population consists of 9 farms with one-yearold micro-basins and 6 farms with five-year-old micro-basins. The walls of all micro-basins are built from soil and stones and the vegetation concerns olive trees. The one-year-old micro-basins are located on Cambisol, Kastanozem, and Arenosol soils; the profiles are characterized as follows: (i) the Cambisols with a very fine textured cambic horizon; (ii) the Kastanozems with a mollic horizon with a moist chroma value 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 least 100 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 or petrocalcic horizon within 100 cm of the surface. For each micro-basin and its associated micro-watershed, measurements are carried out to evaluate vegetation, soil tillage and slope, and surface characteristics (Table 1). The geographical position of the study area is given in Fig. 2; the UTM 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 of soil hydraulic data for the management of olive trees in a hilly semi-arid area. All collected measurements are presented, but the discussion mainly regards the water content values, therefore the infiltration data has only been analyzed in a qualitative way in this paper. However, information about texture and structure collected by evaluating the soil hydrological behaviour through falling head tests, can lead, by knowing the initial water content measurements conditions, to estimate the full set of retention and hydraulic conductivity curves of the considered soils and use physically based simulation models (e.g., Lassabate`re et al., 2006). 2.2.1. Soil parameters Two pits of 1 m depth (one inside and one outside the soil and stone wall) are dug for every micro-basin to examine the pedological profiles. Soil texture, soil structure, skeleton, colour, and organic matter are analyzed at three different depths. The results are shown in Table 2. 2.2.2. Soil water content Measurements are carried out by an ensemble of a three-wire probe connected to a Tektronix 1502C TDR cable tester using a 50 V coaxial cable and a BNC connection. The Roth et al. (1990)
Table 1 Micro-basin locations and relative morphologic data. Farm no.
Soil type
Latitude
Longitude
Slope (%)
Vegetation cover (%)
Tillage
Catchment area (m2)
Micro-basin area (m2)
1 2 3 4 5 6 7 8 9 11 12 13 14 15 16
Cambisols Cambisols Kastanozems Arenosols Calcisols Calcisols Cambisols Kastanozems Calcisols Arenosols Calcisols Kastanozems Calcisols Calcisols Calcisols
568,055 571,371 577,001 568,342 55,2852 568,622 568,567 576,781 553,004 567,537 555,357 576,715 555,831 555,561 552,825
3,949,631 3,953,207 3,948,214 3,942,826 3,911,191 3,951,355 3,942,923 3,947,481 3,911,052 3,941,074 3908596 3,947,598 3,914,163 391,4424 3,911,421
33 27 14 21 16 26 9 19 21 17 18 10 12 5 15
20 15 5 2 5 10 2 20 10 1 5 20 10 10 10
No Outside (old) No No Inside (superficial) Inside and outside No No Inside No Outside (recent) Inside Inside No No
70 36 51 33 63 76 346 51 35 79 35 48 350 40 410
18.4 12.5 29.3 29.0 32.3 28.1 27.3 10.8 22.8 29.0 13.8 18.5 7.0 7.3 28.5
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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 2 Texture 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 Farm no. Soil depth (cm) 0–15 Sand % Silt % Clay % O.M. %
Kastanozems
Arenosols
Calcisols
1
2
7
3
8
13
4
11
5
6
9
12
14
15
16
39 37 24 1.8
31 28 41 0.9
83 5 12 0.0
27 53 20 2.4
51 28 21 2.5
63 20 17 0.6
87 5 8 1.1
93 5 2 0.0
71 18 11 1.3
40 35 25 1.1
73 16 11 0.0
75 13 12 0.0
54 28 18 0.7
57 31 12 1.7
77 21 2 0.8
30
Sand % Silt % Clay % O.M. %
39 37 24 1.8
31 28 41 0.9
37 13 50 0.2
27 53 20 2.4
51 28 21 2.5
63 20 17 0.6
95 3 2 0.3
97 2 1 0.0
38 29 33 0.8
40 35 25 1.1
76 13 11 0.2
75 13 12 0.0
61 27 12 0.5
73 20 7 0.0
77 21 2 0.8
60
Sand % Silt % Clay % O.M. %
27 47 26 1.3
13 35 52 0.9
37 13 50 0.0
BED ROCK
BED ROCK
BED ROCK
97 1 2 0.2
96 2 2 0.1
36 39 25 0.7
39 34 27 0.5
76 13 11 0.2
75 15 10 0.1
69 19 12 0.1
73 20 7 0.0
67 29 4 0.0
relation is used for calculating water content from permittivity. The probes are made of two stainless steel rods (15 cm in length) of 0.5 cm diameter spaced by a nylon spacer at a distance of 5 cm; this to respect the 1/10 ratio between diameter and distance as suggested by Knight (1992). For every soil profile, soil water content measurements are taken inside and outside the 15 micro-basins by the use of a set of two-rod TDR probes inserted into the soil at the depths: 0–15 cm vertically, and 30 and 60 cm horizontally. The survey is conducted over two different time periods: spring 2003 (February–April) and autumn 2003 (October–December). 2.2.3. Infiltration tests Falling head single ring infiltrometer tests (e.g., Lassabate`re et al., 2006; Touma et al., 2007) are carried out by the use of a PVC cylinder (11 cm in diameter) slightly inserted into the soil and filled with 10 cm of water height. The infiltration time is recorded every centimeter of falling water head until all water is infiltrated. Infiltration tests are carried out inside and outside the 15 microbasins giving a total amount of 30 measurements.
3. Results and discussion 3.1. Soil water content The soil water content measurements are analyzed as a function 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 at the one-year-old micro-basins, whereas the Calcisols are found for the five-year-old micro-basins. Water content values are measured simultaneously inside and outside the micro-basins. The results are presented in Fig. 3. Two time periods are chosen for the measurements: spring time and autumn. The overall comparison between the inside and outside measurements shows that the water content values measured inside the micro-basins are generally higher than those monitored outside the micro-basins. However, and in spite of this general tendency, some noticeable effects due to soil type and time period are observed.
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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 3 Average water content values ðu¯ Þ 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
Cambisols
Kastanozems
Arenosols
Calcisols
a
Location
Average values
Spring
Spr.
Aut.
Overall
Farm 1 Farm 2 Farm 7
Ins. Out. Ins. Out. Ins. Out.
0.271 0.228 0.352 0.194 0.294 0.258
0.377 0.308 0.460 0.253 0.439 0.325
0.324 0.268 0.406 0.224 0.367 0.291
Farm 3 Farm 8 Farm 13
Ins. Out. Ins. Out. Ins. Out.
0.197 0.176 0.068 0.061 0.224 0.122
0.161 0.192 0.122 0.142 0.186 0.256
0.179 0.184 0.095 0.101 0.205 0.189
Farm 4 Farm 11 Farm 5 Farm 6 Farm 9 Farm 12 Farm 14 Farm 15 Farm 16
Ins. Out. Ins. Out. Ins. Out. Ins. Out. Ins. Out. Ins. Out. Ins. Out.
0.068 0.041 0.080 0.071 0.173 0.134 0.088 0.068 0.072 0.061 0.200 0.113 0.195 0.117 0.197 0.186 0.183 0.151
0.177 0.152 0.154 0.116 0.138 0.196 0.250 0.207 0.170 0.125 0.076 0.182 0.223 0.185 0.100 0.104 0.129 0.132
0.123 0.096 0.117 0.094 0.155 0.165 0.169 0.138 0.121 0.138 0.148 0.209 0.151 0.149 0.145 0.156 0.141
Autumn
General
Ins.
Out.
Ins.
Out.
Ins.
Out.
0.306
0.227
0.425
0.295
0.366
0.261
0.163
0.120
0.156
0.197
0.160
0.158
0.074
0.056
0.165
0.134
0.120
0.095
0.158 0.147a
0.155 0.117a
0.118 0.175a
0.162 0.151a
0.138 0.161a
0.158 0.134a
Without considering the farms 5 and 12 (which present incorrect labour practices) the average values show the proper micro-basin functions.
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Fig. 4. Surface water storage action on a Cambisol micro-basin during a rainy day (February 28, 2003).
The results obtained from the Cambisols (Fig. 3a) are the most interesting. At some points, an increase in water stock of 30% is measured. As these soils are characterized by the presence of fine materials over great depths, they exhibit a good water retention capacity combined with high water storage. It explains why the values of the volumetric water content are bigger than those observed for the other soil types. Calculating the average water content value (u¯ ) for respectively the autumn and spring data series of both the inside and outside measurements (Table 3), the Cambisol u¯ -values are systematically the biggest. Denoting the ‘‘inside autumn’’ measurements with the suffix ‘‘ins-aut’’, u¯ ins-aut ¼ 0:425 cm3 =cm3 for the Cambisols against u¯ ins-aut ¼ 0:156 cm3 =cm3 for the Kastanozem soils, u¯ ins-aut ¼ 0:165 cm3 =cm3 for the Arenosols, and u¯ ins-aut ¼ 0:175 cm3 =cm3 for the Calcisols. The differences between inside and outside measurements are particularly visible during the autumn period when rains are more abundant. During spring time the measurements taken outside the micro-basins benefit of some rare rainfall events. This explains why the difference between u¯ ins-aut and u¯ out-aut is more important than that observed for u¯ ins-spr and u¯ out-spr (where the suffix ‘‘spr’’ refers to the spring period). For illustration, Fig. 4 shows the surface storage on a Cambisol micro-basin after a rainfall event. The Kastanozems (Fig. 3b) are loamy textured fertile soils with a high quantity of organic matter. Unfortunately, the Kastanozem soils presented in this study form a very thin soil layer. Consequently, the water storage capacity is strongly reduced. For these soil profiles, the use of micro-basins is not advisable as no strong benefit can be expected. This is clearly demonstrated by the average values of
453
u¯ ins-gen and u¯ out-gen which are quasi identical, e.g., u¯ ins-aut ¼ 0:160 cm3 =cm3 and u¯ out-aut ¼ 0:158 cm3 =cm3 . When comparing the two time periods, the autumn data show a big scatter which is difficult to explain a priori. The spring data are more consistent. The results obtained for the Arenosols (Fig. 3c) are similar to those observed for the Cambisols. The autumn data show a much wetter soil profile inside than outside the micro-basins with u¯ ins-aut ¼ 0:165 cm3 =cm3 and u¯ out-aut ¼ 0:134 cm3 =cm3 . According to the U.S. Department of Agriculture (Soil Survey Laboratory Staff, 1992) the particle-size distribution of Arenosols is mainly composed of sand particles with a diameter between 0.02 and 2 mm (7th USDA Soil Classification system). Consequently, the water holding capacity of the Arenosols is strongly reduced and the numerical value u¯ ins-aut ¼ 0:165 cm3 =cm3 is small as compared with that of the Cambisols u¯ ins-aut ¼ 0:425 cm3 =cm3 . Once again (except Kastanozems, as explained before), the differences between inside and outside measurements are particularly visible for the autumn period when rains are scarce. The last class of soils concerns the Calcisols (Fig. 3d). For both spring and autumn time, the inside measurements are wetter than those observed outside the micro-basin, e.g., u¯ ins-aut ¼ 0:175 cm3 =cm3 and u¯ out-aut ¼ 0:151 cm3 =cm3 . However, as will be shown below, the results obtained for this soil type are particularly sensitive to the farm management such as tillage practices. Resuming, the above results show that the use of micro-basins has a positive effect on the water holding capacity of the soil profile especially for fine textured soil types such as the Cambisols. The time period also has an influence with the micro-basins operating more efficiently during the less frequent rainfall periods of autumn. 3.1.2. Farm management As to the influence of farm management, the effect of tillage is analyzed next. The five-year-old micro-basins are chosen for these tests, because they are all built on the same type of soil (Calcisols). The results of two farms (no. 6 and no. 12) are selected for comparison. Generally speaking, tillage practices such as ploughing, or scarifying, enhance the fine textured soil profiles and avoid the progressive formation of calcipans (hard impermeable crust layers) over time. Hence, when tillage practices are carried out correctly, farm management has definitely a positive effect on the water storage capacity of soils (Fig. 5a, farm no. 6). However, when the tillage practices are carried out incorrectly the results are rapidly deteriorating. The example is shown in Fig. 5b, observed for farm no. 12. During the spring period, the micro-basin functions
Fig. 5. Comparison between correct tillage practices (a) observed for farm no. 6 and incorrect farm management measured for farm no. 12 (b).
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perfectly enhancing soil water storage in the soil profile. Due to successive wetting and drying, crust forming takes place at the soil surface sealing off slowly but surely the soil profile. This effect is clearly put in evidence by the autumn measurement. While the outside water content measurements react normally to rainfall (no crust forming has taken place outside the micro-basins), the soil profile inside the micro-basin is drying out due to the fact that infiltration is blocked off by the crust layer at the soil surface. The above results are extremely illustrative. They show the importance of accompanying micro-basin practices with appropriate farm management. Since water clogging within the microbasins have the tendency to generate crust formation for most soil types (and not only for the Calcisols of the above example), the maintenance of the soil surface is one of the prime conditions necessary for successful water harvesting by the use of micro-basin practices. Associated with the problem of crust formation, there is a problem of soil erosion and soil conservation. During harvesting of rainwater, small soil particles are transported over the slope surface to the micro-basin. This erosion process generally concerns small particles with a diameter smaller than 0.2 mm. When estimating the erosion losses for the different farms by the use of the USLE-method (Wischmeier and Smith, 1978), a volume of 2– 100 t ha1 y1 is calculated. This volume is partially discharged within the micro-basin. An example is given in Fig. 6. Once again, tillage inside the micro-basins is the recommended farm practice to overcome the problem of soil erosion in relation with its positive effects on water infiltration enhancements which leads to the deposition of soil particles eroded on the slopes. 3.1.3. Soil depth The last test concerns the effect of soil depth. For this test, the ensemble of water content measurements is classified in three groups as a function of soil depth. The first group of data concerns the top layer with z < 15 cm; the second data set refers to the layer z = 30 cm; and the last group applies to z = 60 cm. The results are shown in Fig. 7a–c. The soil water storage efficiency increases with depth. Water storage at deeper depths is important as it allows water storage over longer periods. This is particularly efficient when evaporation losses are avoided. 3.2. Infiltration behaviour When carrying out infiltration experiments inside and outside the micro-basin, different infiltration dynamics are observed as a function of soil type. The time required to infiltrate 10 cm of water is plotted in Fig. 8.
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.
Fig. 6. The effects of surface erosion into a sandy soil micro-basin.
The comparison between the time intervals observed inside and outside the micro-basins, shows for nearly all farms a longer infiltration time measured inside the micro-basin. This delay is certainly related to the fact that the water harvesting trails fine material in suspension. Once discharged within the micro-basin, it slightly modifies the textural composition of the surface layer. The
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Special thanks go out to the Tunisian Headquarters, their whole staff, and the Kairouan displace unit. Our gratitude also goes to the reviewers for their timely and accurate reviews and for their help in improving this work. References
Fig. 8. Infiltration time necessary to infiltrate 10 cm of water head measured inside and outside the micro-basins of every single farm.
finer the soil material, the slower the infiltration dynamics and hence the longer the time period to infiltrate 10 cm of water head. The Arenosols show fast infiltration behaviour. This is explained by the fact that the sandy soils are characterized by relatively high permeability in the absence of fine material. As the Kastanozem layer is thin, the infiltration dynamics observed for this soil type rather reflect the infiltration behaviour of the underlying sand matrix. 4. Conclusions In this study micro-basin slope management for rainfall harvesting is evaluated. To do so, water content measurements inside and outside the micro-basins are compared for different soil types. The overall comparison between the inside and outside measurements shows that the water content values measured inside the micro-basins are generally higher than those monitored outside the micro-basins. However, and in spite of this general tendency, 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 microbasins have an increasingly important effect for fine textured soils. When comparing the yearly averaged differences of the inside minus outside water content data taken in the soil surface layer (z 15 cm), an increase of 0.10 cm3/cm3 is observed for fine textured soils (i.e., Cambisols) against only 0.03 cm3/cm3 for sandy soils (Arenosols). The time period of the year also has an influence. The less frequent the rain is (e.g., the autumn period) the more efficiently operate the micro-basins. But among all, farm management is shown to be one of the most important key factors to succeed water harvesting by the use of micro-basins. Tillage practices not only avoid crust forming at the soil surface, but also enhance dry-mulching effects which may become increasingly effectual for deeper layers of the soil profile. Obviously, the above list of key factors affecting the efficiency of water harvesting by the use of micro-basins should not be seen as exhaustive. Other effects, such as rainfall intensity and duration, catchment size, stones and rocky surfaces, also strongly influence the 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-mentioned FAO Project and the Italian Ministry of Research through Project PRIN 2007: ‘‘Experimental measurements of the atmosphere–vegetation–soil interaction processes and their response to climate change’’.
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