Evaporation losses from bare soils as influenced by ......Evaporation losses from bare soils as...

Evaporation losses from bare soils as influenced by

cultivation techniques in semi-arid regions

H.J. Melloulia, B. van Wesemaelb,*, J. Poesenc, R. Hartmannd

aInstitut National de la Recherche Agronomique de Tunisie, Rue HeÂdi Karray, 2049 Ariana, TunisiabFlood Hazard Research Centre, Middlesex University, Queensway, EN3 4SF Enfield, UK

cFund for Scientific Research Flanders, Laboratory for Experimental Geomorphology, K.U. Leuven,

Redingenstraat 16, 3000 Leuven, BelgiumdFaculty of Agricultural and Applied Biological Sciences, Universiteit Gent,

Coupure Links 653, 9000 Ghent, Belgium

Accepted 25 February 1999

Abstract

The impact of cultivation techniques on the evaporation from bare soils was investigated in the

laboratory. Two soil-types, which are important resources for rainfed cultivation of olives and

almonds in semi-arid regions, were selected: a loamy sand soil and a stony (loam) soil. Evaporation

from the soil surface is an important loss of soil moisture in these farming systems since a large

percentage of the soil is kept bare in order to maximise the water availability for the tree crop. For

the loamy sand soil the impacts of a straw mulch and treatment of the topsoil with olive mill

effluent (OME) were tested. For the stony soil the effects of different rock fragment contents and

distribution within the soil profile were tested. After thoroughly wetting with simulated rainfall and

allowing the soil moisture to redistribute, the columns were subjected to evaporation for 46 days.

Cumulative evaporation depth of soils treated with OME was 28% lower than that of the control

soil. A similar reduction, be it lower (16%) was observed for the soil with a high rock fragment

content by volume (Rv � 0.35 m3 mÿ3). The straw mulch and rock fragment mulch did not have an

impact on the cumulative evaporation depth after 46 days. Furthermore, the time required to reach

half of the total evaporation losses (d0.5) increased from 9 days for the control soil (loamy sand) to

24 days for the soil impregnated with OME and to 15 days for the straw mulch treatment. The same

trend was observed for the stony soils: an increase in d0.5 from 4 days for the control soil

(Rv � 0.19 m3 mÿ3) to 7 days for the soil with Rv � 0.35 m3 mÿ3 and to 8 days for the rock

fragment mulch. These experiments show that the changes in water retention capacity of the topsoil

by treatment with a hydrophobic substance (OME) or an increase in rock fragment content have a

Agricultural Water Management 42 (2000) 355±369

* Corresponding author. Tel.: +44-181-362-6611; fax: +44-181-362-6957E-mail address: [email protected] (B. van Wesemael)

0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 4 0 - 2

longer lasting effect on the reduction of evaporation losses, and result in a higher and more evenly

distributed soil moisture content. # 2000 Elsevier Science B.V. All rights reserved.

Keywords: Evaporation losses; Cultivation techniques; Topsoils treated with olive mill ef¯uent; Stony soils;

Semi-arid regions

1. Introduction

Evaporation from bare soils results in a considerable loss of moisture and has a direct

impact on crop yield in rainfed agriculture of arid and semi-arid regions. Under annual

field crops the soil surface remains bare for many weeks when the moisture content of the

upper soil layer can be of critical importance during periods of seed germination and

seedling establishment as well as during the subsequent growth of the young crop. In

orchards, a typical component of agriculture in the Mediterranean region, the soil surface

between the trees is kept bare by frequent tillage and is continuously subjected to

evaporation. The very wide spacing between the trees allows a maximum catchment and

soil volume to collect and store the sparse rainfall for individual trees (spacing between

the trees extends to 24 m; Ennabli, 1993).

In the U.S. Midwest up to 50% of the maize evapotranspiration is lost by evaporation

during a normal growing season (Peters, 1960). Research conducted in Tunisia for two

decades demonstrated that evaporation from bare soils accounted for 75% of the annual

precipitation (Riou, 1977). Under similar climatic conditions, this water loss was

evaluated to be 30±61% of the annual precipitation for various rainfed winter wheat-

based production systems (Stewart and Burnett, 1987). By means of an evaporation

simulation model, Floret et al. (1982) estimated that in the south Tunisian steppe region

where the soil surface cover of perennial grasses (Stipa spp. ) is about 30%, half of the

available soil moisture is lost by evaporation from bare soil. In Syria, where soil nutrient

deficiency often severely limits crop growth, as much as 75% of the soil moisture is lost

by evaporation from the soil surface under barley (Cooper et al., 1987). Since these

evaporation losses are very important, their reduction will strongly contribute to soil

moisture conservation for crop production. This is even more urgent in arid and semi-arid

regions, characterised by highly variable and often chronically deficient rainfall. Many of

the countries in these regions will have to take up an important challenge at the beginning

of the next century: increasing food production in order to realise food security for a

growing population while optimising the use of limited water resources (FAO, 1994). In

addition, climate change will affect rainfall depth and its reliability which will increase

the threat of desertification.

Management techniques commonly called `mulching' are mostly based on the

modification of the soil surface conditions. Mulching enhances the formation of thin air-

dry layer on the top bare soil, which hampers capillary rise and slows the evaporation

process. Such a well-developed air-dry surface layer can either be achieved by frequent

tillage, commonly called dry farming (Willis and Bond, 1971; Jalota and Prihar, 1990a;

Mwendera, 1992) or by the application of a soil cover (e.g. straw, sand, gravel or plastic

film). The role of various types of soil cover in reducing evaporation has been widely

356 H.J. Mellouli et al. / Agricultural Water Management 42 (2000) 355±369

studied: crop residue mulches (Bond and Willis, 1969; Unger and Parker, 1976; Smika,

1983; Tanaka, 1985; Jalota and Prihar, 1990b; Gicheru, 1994; Mellouli, 1996), plastic

film (e.g. El Amami and Haffani, 1974), sand mulches (Modaihsh et al., 1985; Bousnina,

1993) and rock fragment mulches (Corey and Kemper, 1968; Groenevelt et al., 1989;

Kemper et al., 1994; van Wesemael et al., 1996). However, these mulches are labour-

intensive to apply and maintain (Gale et al., 1993) and increasingly crop residues have

become a valuable food supply for livestock.

Other techniques to reduce evaporation from bare soils are based on modification of

the hydrodynamics of the topsoil (i.e. to force water to penetrate deep into the soil

profile). Both the aggregate stability of the topsoil and its rock fragment content are

important factors in this respect. The increase in structural stability of the topsoil assists

in the rapid development of a dry mulch layer and can be achieved by mixing the topsoil

with a hydrophobic compound either of synthetic origin (e.g. soil conditioners, Hillel and

Berliner, 1974; Al-Jaloud, 1988; Verplancke et al., 1990) or of natural origin, for

example, Olive Mill Effluents (OME; Mellouli, 1996; Mellouli et al., 1998). OME, better

known in Tunisia as `margines', that is, liquor from olive oil extraction processes, are

characterised by their fertility properties (e.g. Fiestas Ros de Ursinos, 1986), as well as by

their adhesive and hydrophobic behaviour (e.g. FriaaÃ et al., 1986). Mellouli (1996) has

shown that OME has a beneficial influence on soil aggregation, soil structure stability and

hydrodynamic properties of a sandy soil, for example, liquid±soil contact angle (water

repellency) and saturated hydraulic conductivity were increased. Furthermore, unsatu-

rated hydraulic conductivity and water retention at the pressure potential range

corresponding to the available water capacity can be reduced by the application of

OME.

The main impact of the rock fragment content in stony soils on evaporation is

attributed to the reduction of the water holding capacity and deeper penetration of water

in stony soils compared to non-stony soils (Ravina and Magier, 1984; Kosmas et al.,

1993; Poesen and Lavee, 1994; van Wesemael et al., 1995, 1996). Stony soils are quite

common in semi-arid regions. They cover more than 60% of the land in the

Mediterranean (Poesen and Lavee, 1994). Frequent tillage of, in particular, orchards

leads to the development of a spatial pattern of stoniness on hillslopes (Poesen et al.,

1997). Denudation creates very stony soils on the convexities whereas kinetic sieving and

the downward transport of rock fragments by the tines of the chisel ploughs result in a

rock fragment mulch at the foot of the slopes.

The overall objective of this paper is to investigate the impact of different cultivation

practices with regard to water conservation in two soils which are important resources for

dryland farming where a large proportion of the soil remains bare: (i) a loamy sand soil

typical for a large proportion of the olive plantations in the coastal plains on the southern

fringes of the Mediterranean and (ii) a stony soil typical for many areas of rainfed

agriculture in the uplands which are increasingly used for tree crops such as almonds and

olives (Fig. 1). The specific research questions are:

1. Whether physical modifications of the upper few centimetres of a loamy sand, by

incorporating OME will create a stable and hydrophobic mulch and will have a

favourable effect on the soil water balance by reducing the evaporation.

H.J. Mellouli et al. / Agricultural Water Management 42 (2000) 355±369 357

Fig. 1. Two typical examples of farming systems where a large proportion of the soil is bare throughout the year.

(a) olive groves near Sfax (Tunisia) with a loamy sand soil and an annual precipitation of 190 mm and (b)

almond plantations in the Guadalentin basin (south-east Spain) on stony soils with an annual precipitation of

280 mm.


2. Whether modification of moisture retention capacity in loamy soils by the increase of

rock fragment content (resulting from intensive cultivation on hillslopes) will force

rain water to penetrate deeper in the profile where it should be saved from the

evaporation process.

2. Methods

Laboratory experiments were conducted in order to simulate two conditions that are

common in rainfed agriculture within semi-arid Mediterranean landscapes: loamy sand

soils and stony soils (Figs. 1 and 2).

2.1. Loamy sand soil

The soil used in this experiment is a loamy sand containing 7.9% clay, 16.2% silt and

75.9% sand, with an organic matter content of 1.7%. The OME used in the treatment of

the soil has the following characteristics: pH(H2O) � 5.0; electrical conductivity:

10 mS cmÿ1; density: 1.034 Mg mÿ3; dry matter content: 10.5%; mineral composition:

1%; organic matter: 9.5%. Three soil containers (length � width � height: 48.5 cm �48.5 cm � 50 cm), having a drainage system, were filled for 45 cm from the bottom with

the soil compacted to a bulk density of 1.4 Mg mÿ3. The uppermost 5 cm of these

containers were treated in the following way (Fig. 2):

1. Surface layer of untreated aggregates (0±8 mm) simulating the natural mulch situation

(control);

2. Similar to the `Control' except for a surface cover of straw. Straw was applied at the

optimal rate of 450 g mÿ2 (Bond and Willis, 1969; Smika, 1983);

3. Surface layer of aggregates (0±8 mm) impregnated with OME at a concentration of

1% of active material on dry soil basis as suggested by Mellouli (1996).

The soils were first exposed for 3 h to a rainfall intensity of 20 mm hÿ1 by means

of a rainfall simulator in order to investigate how the different surface conditions

affect water infiltration and redistribution (Kamphorst, 1987; Fig. 2). Then the surfaces

of the containers were covered during 3 days to allow redistribution of soil moisture.

The containers were subsequently subjected to evaporation for 46 days (corresponding

to a cumulative potential evaporation (Ep) of 460 mm) in a controlled environment

simulating the evaporation conditions of Mediterranean regions (Potential evaporation

rate ep � 9.9 � 1.8 mm dayÿ1, temperature fluctuated between 20±308C and relative

humidity between 50±85%). Moisture content was measured by means of Time Domain

Reflectometry (TDR). Two-rod probes were inserted horizontally in the containers

at 2.5 cm depth in the surface layer and at 3 cm interval from 6.5 cm depth. Drainage

was not observed and therefore evaporation was determined by change in soil

moisture storage. Soil moisture storage is defined by the analytical integral, from

the soil surface to a certain depth z, of the smooth moisture content profile, �(z). Since

no analytical integral is available, the storage was obtained by a numerical (trapezoidal)

method.


2.2. Stony soil

The treatments were selected to represent a common scenario induced by tillage of

shallow, stony soils: a stony control soil with a rock fragment content by volume (Rv) of

0.19 m3 mÿ3 that can evolve either to a non-stony soil with a rock fragment `mulch' on

top by kinetic sieving of the rock fragments in case of shallow tillage (Oostwoud

Fig. 2. Soil columns used for the experiments with loamy sand and stony soils.


Wijdenes et al., 1997; Poesen et al., 1997) or to a skeletal soil with an increased rock

fragment content (e.g. Rv � 0.35 m3 mÿ3) by breaking up the bedrock and bringing the

rock fragments to the surface (Fig. 2). Soil columns with a diameter of 19 cm and a

height of 20 cm were filled with an aggregated silt loam soil containing rock fragment

contents by volume (Rv) of 0.19 and 0.35 m3 mÿ3. In addition, a 5 cm thick layer of rock

fragments was applied to a similar silt loam soil (Fig. 2). Each treatment was duplicated.

The fine earth material consisted of well-structured, homogeneous silt loam (13% clay,

78% silt, 9% sand and 0.4% organic matter). Its dry aggregate size distribution was close

to that of a fine seedbed: 14% of the aggregates was smaller than 2 mm, 46% was

between 2 and 11.2 mm, 30% was between 11.2 and 31.5 mm and 10% was larger than

31.5 mm. Well-rounded river gravel (mainly flint and quartzites) were sieved and only the

fraction between 1.7 and 2.7 cm was used. The soil moisture content of the fine earth

equalled 0.18 kg kgÿ1.

Rainfall (30 mm) was applied to the columns with a rainfall simulator (intensity:

35 mm/h; Poesen et al., 1990). The columns were covered with a fine PVC mesh to

reduce degradation of the soil surface. The bottoms of the columns were perforated to

drain excess moisture. The columns were covered with a plastic sheet for 2 days in order

to drain excess moisture and to start the experiment with equilibrated moisture

conditions. Thereafter, the bottom of the columns was closed, the plastic cover was

removed, the columns were weighed and placed on a drying table with two fans blowing

across. After weighing the columns daily, they were replaced in a different position.

Maximum and minimum temperature and relative humidity were monitored on a daily

basis. Three similar columns filled with water were used to measure potential evaporation

(Ep). Potential evaporation varied around a mean of 7.7 mm dayÿ1 (5.6±10.1 mm dayÿ1)

due to variations in mean temperature 20.18C (16±288C) and relative humidity (Rh)

77.8% (58±95%)

3. Results and discussion

3.1. Loamy sand soil

The initial soil moisture distribution with depth was more or less equal in all containers

(Fig. 3(a)). At the start of the evaporation experiment (3 days after infiltration) the

amount of soil moisture in the three columns was equal although a more evenly

distributed moisture profile was obtained in the case of the OME treatment (Table 1;

Fig. 3(b)). This can be attributed to the fact that the aggregates, present at the soil surface,

have acquired hydrophobic and adhesive properties due to their treatment with OME.

Aggregate stability increases due to OME treatment while the aggregates themselves do

not take up moisture which leads to a higher infiltration rate and a deeper penetration of

rain water in the soil through the large cavities between the stable aggregates (Mellouli,

1996; Mellouli et al., 1998). These processes lead to a decrease in water retention in the

upper 20 cm of the soil profile and a more homogeneous distribution of the water content

within the entire profile.


Fig. 3. Soil moisture distribution profiles of the columns used for the experiment with loamy sand soil: (a)

Initial soil moisture content profiles, (b) soil moisture profiles after 60 mm rainfall and redistribution of soil

moisture for 3 days, and (c) soil moisture content profiles after 46 days of evaporation.

After 46 days, reduction of the cumulative evaporation of the OME treatment

compared to the control treatment amounted to 27.9%. The straw mulch did not seem to

be effective since the evaporation reduction was limited to 4.5% of the control (Table 1).

The cumulative evaporation of the different soil columns as a function of the cumulative

potential evaporation (Ep), the environmental evaporative demand, is plotted in Fig. 4(a).

Two phases in the evaporation process can be clearly distinguished: a first phase with a

more or less constant evaporation rate which is energy-limited and a second phase with a

decreasing evaporation rate which is soil moisture content limited (e.g. Black et al.,

1969). The OME topsoil is the most efficient in reducing evaporation losses and affects

both the first and second phase (Fig. 4(a)). Where straw was applied as a mulch, the

whole system reacted as a two-layered mulch. The application of straw is very efficient in

the first stage of evaporation but has no effect in the second phase and after 46 days the

effect is negligible since the length of the first phase is extended. The restriction of the

efficiency of mulches to the constant rate stage was already observed by Corey and

Kemper (1968) and Groenevelt et al. (1989).

The evaporation retarding effect of the two treatments on water conservation can be

observed from the potential evaporation required to lose about one-half of the total

cumulative evaporation of the control treatment (Ep 0.5). The Ep 0.5 equals 74 mm

(reached in 9 days) for the control experiment, 134 mm (reached in 15 days) for the straw

mulch and 232 mm (reached in 24 days) for the OME topsoil (Table 1; Fig. 4(a)).

The presence of a topsoil treated with OME has a beneficial effect on the soil moisture

content distribution after 46 days of evaporation (Fig. 3(c)). A homogeneous moisture

content along the entire soil column at a higher level than that of the control or straw

experiment were observed. Such a higher moisture content within the plough layer is

important for germination and the development of young plants. A homogeneous

distribution of water and its conservation over the upper 50 cm of the profile is also

beneficial for the development of some deep rooting crops.

Table 1Initial and final soil moisture storage and effects of the different treatments on cumulative evaporation at the endof the experiment (E46) and at the time when one-half of the final amount of cumulative evaporation is lost (E0.5

and d0.5)

Loamy sand soils Stony soils

Control Straw OME Control RF mulch Rv � 0.35

Initial moisture (mm) 115.6 117.4 117.8 48.4 45.2 39.7

Final moisture (mm) 57.9 62.3 76.2 5.2 2.8 3.4

E46 (mm) 57.7 55.1 41.6 43.2 42.4 36.3

�E46/E46control (%) 0 4.5 27.9 0 1.9 16.0

Ep 0.5 (mm) 74 134 232 34 62 55

d0.5 (days) 9 15 24 4 8 7

OME, Treatment with olive mill effluent.E46, Cumulative evaporation at the end of the experiment (46 days).�E46/E46control, Relative efficiency in E46 reduction compared to the control experiment.Ep 0.5 Potential evaporation at the time when half of the E46 is lost from the control experiment (30 mm for

loamy sand soils and 22 mm for stony soils).d0.5 Length of period to reach Ep 0.5.


3.2. Stony soil

The soil in the cylinders has compacted somewhat due to the impact of the raindrops

resulting in a soil depth of 15.9 cm for Rv 0.19 m3 mÿ3, 16.9 cm for Rv 0.35 m3 mÿ3 and

17.0 cm for the rock fragment mulch (Fig. 2). The difference in initial soil moisture

Fig. 4. Effect of different treatments on cumulative evaporation (E) as a function of evaporative demand (Ep) for

(a) the loamy sand soil and for (b) the stony soil.


content between the three treatments can be explained by the decrease in water holding

capacity due to an increased volume taken up by rock fragments and a higher macro-

porosity of soils containing rock fragments (Childs and Flint, 1990; van Wesemael et al.,

1995). The volume of the column taken up by rock fragments is similar for control

(Rv � 0.19 m3 mÿ3) and the rock fragment mulch. This is reflected in similar initial soil

moisture contents (Table 1). The column with the highest rock fragment content

(Rv � 0.35 m3 mÿ3) has the lowest water-holding capacity and therefore the lowest

moisture content at the start of the evaporation experiment.

After 46 days, the cumulative evaporation of the column with Rv � 0.35 m3 mÿ3 is

16% less than the control. The rock fragment mulch did not seem to be effective since the

evaporation reduction was limited to 1.9% of the control (Table 1). The column with a

higher rock fragment (Rv � 0.35 m3 mÿ3) shows a shorter constant rate stage evaporation

and a lower evaporation rate thereafter compared to the control experiment

(Rv � 0.19 m3 mÿ3; Fig. 4(b)). This decrease in the evaporation rate can be explained

by the reduced water-holding capacity of stony soils resulting in drainage of excess

moisture. The rock fragment mulch results in a lower evaporation rate during the constant

rate stage, although the constant rate stage lasts longer and the final cumulative

evaporation is similar to that of the control experiment.

Similar to the experiment with the loamy sand soils, the evaporation retarding effect of

the various treatments on water conservation can be observed from the potential

evaporation required to lose about half of the total cumulative evaporation of the control

treatment (Ep 0.5). The Ep 0.5 equals 34 mm (reached in 4 days) for the control experiment,

62 mm (reached in 8 days) for the rock fragment mulch and 55 mm (reached in 7 days)

for the Rv � 0.35 m3 mÿ3 (Table 1).

4. The role of cultivation practices in reducing evaporation losses

Most efforts to prevent moisture loss by evaporation from bare soils have been directed

towards modification of the soil surface conditions. The experiments in this paper have

shown that straw and gravel applied on top of the soil act as two-layered mulches.

However, their efficiency is short-lived, mainly restricted to the constant rate stage

of the evaporation process (e.g. Groenevelt et al., 1989). However, these mulches

tend to lengthen the constant rate stage and therefore on the longer term, cumulative

evaporation is similar to that from the control experiments. These mulches are therefore

best applied in situations where crops are very sensitive to drying during the initial

evaporation phase (e.g. germination of young plants or irrigated crops). The high level

of labour and investments involved in the application and maintenance of these mulches

as well as the scarcity of crop residues in semi-arid regions limits their extensive use

(Gale et al., 1993).

The experiments in this paper show that the modification of the hydrodynamic

properties of the plough layer has a longer lasting effect on water conservation compared

to surface mulching and is therefore more relevant to rainfed agriculture in semi-arid

regions. The impregnation of the loamy sand soil with hydrophobic substances such as

OME at a concentration of 1% of active material on a dry soil basis increases their


aggregate stability and reduces their unsaturated hydraulic conductivity (Mellouli, 1996).

Treating the upper 5 cm of soil columns with OME resulted after 46 days of evaporation

in a homogeneous distribution of water and its conservation over the upper 50 cm. In

particular, tree crop plantations could benefit from such conservation of water over a

prolonged period, since a large proportion of the surface is kept bare between the trees

and the water availability for each tree is stored in a large volume of soil (e.g. Ennabli,

1993). Disposal of OME in the sewage system poses considerable problems because of its

high organic matter and polyphenol content. However, trials in Spain and France with the

spreading of OME at a rate of not more than 100 m3 haÿ1 have proven that the

polyphenols can be broken down rapidly and that the high nutrient content of OME can

partly replace fertilisers (Fiestas Ros de Ursinos, 1986). The magnitude of the

evaporation reduction obtained with OME in the laboratory experiments suggests an

additional advantage of the spreading of OME in olive plantations.

On hillslopes with thin soils, rock fragments tend to move to the surface by tillage with

chisel ploughs (Oostwoud Wijdenes et al., 1997; Poesen et al., 1997). Frequent tillage on

slopes will lead to the creation of two types of soil profiles: (a) a rock fragment mulch, if

the tillage depth does not exceed the soil depth or (b) a profile with a high rock fragment

content if the tines of the chisel plough break up the bedrock (Fig. 2). Poesen et al. (1997)

demonstrated that these two types of soil profiles will generally occur in different

landscape positions dictated by slope characteristics that determine the intensity of tillage

erosion. The thin stony soils will occur on the convexities and the deeper soils with a rock

fragment mulch will prevail at the foot of the slopes. From the results of the laboratory

experiments, it is possible to predict the consequences of this spatial distribution of soil-

types for water conservation. The thin stony soils on the convexities will behave similarly

to the column with Rv � 0.35 m3 mÿ3 and will only retain small amounts of soil

moisture. Water conservation will be efficient even in the longer term by reduced

evaporation and redistribution of soil moisture downslope to the deeper soils on the

footslopes and in valley bottoms. These soils are covered by a rock fragment mulch that

to some extent protects soil moisture from evaporation losses at least during the initial

phase. Intensive cultivation with a tine-like implement (e.g. in south-east Spain two-to-

three times per year, Poesen et al., 1997) will thus cause a clear distinction between

runoff generating areas (shallow stony soils) where water losses are limited due to low

soil moisture retention and water receiving footslopes and valley bottoms with a tillage-

induced rock fragment mulch that protects against evaporation.

The results are in agreement with the calculations of Riou (1977), who indicated that

soil water retention characteristics are crucial in the availability of water for the plants in

semi-arid regions. Field experiments are required to assess the costs and benefits of

cultivation techniques such as the incorporation of OME or the increase of rock

fragments' content by frequent tillage.

5. Conclusions

The water-use efficiency of tree crops such as almonds and olives in semi-arid

environments depends to a large extend upon the evaporation rates from the bare soil in


between the widely spaced trees. The expansion of rainfed tree crops in Mediterranean

environments has stimulated this research into the impacts of the modification of surface

and subsurface characteristics on evaporation losses. The results of laboratory

experiments indicate that the effects of surface treatments (a straw mulch for a loamy

sand soil and a rock fragment mulch for a stony soil) are relatively short-lived, whereas

after 46 days (Ep � 10 mm/day) sub-surface treatments (impregnation with OME for the

loamy soil and increase of rock fragment content of the stony soil) significantly reduce

evaporation losses. These results indicate that cultivation techniques should aim to reduce

the water holding capacity in the topsoil. This can either be achieved by treatment with a

hydrophobic substance (OME), or it will result from frequent tillage of stony soils on

hillslopes whereby the stone content increases by breaking up the bedrock and kinetic

sieving. Field experiments into the impacts of the water-holding capacity on water-use

efficiency are in progress.

Acknowledgements

The research for this paper was financed by the Tunisian±Belgian bilateral co-

operation (ABOS) and the European Union. The first author wishes to thank the ABOS

for enabling him to carry out the research in Belgium. The second and third authors

participated in this research as part of the MEDALUS III (Mediterranean Desertification

and Land-Use) collaborative research project. MEDALUS III was funded by the

European Commission Environment and Climate Research programme (contract number:

ENV4-CT95-0118), and this support is gratefully acknowledged.

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