Surface irrigation simulation models have seldom been used in engineering practise, and district modernisation is not an exception. Surface irrigation evaluations were performed in the Almudévar irrigation district to obtain the parameters required for surface irrigation modelling. The total district irrigated area was divided into 92 design units, for which a characteristic blocked-end border was defined. Simulation was used to establish the current irrigation performance in each design unit, and district performance contour maps were built. Irrigation performance was characterised using potential application efficiency, with an average of 54%, and irrigation time, averaging 6 hr ha -1 . Simulated potential application efficiency of the low quarter was similar to the Seasonal Irrigation Performance Index (an estimate of irrigation efficiency) presented in the companion paper. A set of seven modernisation scenarios was defined. Two

Key words:Preparedness measures to cope with droughts and water scarcity in semi-arid climates in irrigated agriculture include the identification of irrigation scheduling strategies that minimize the water demand with acceptable impacts on yields. Those strategies may be produced by simulation and focus on different levels of water demand. The irrigation scheduling simulation model ISAREG, validated for Tunisia, is used to simulate those strategies. The generated irrigation scheduling strategies are applied to deficit irrigation of winter wheat, tomato and potato crops under semi-arid conditions in Central Tunisia. The alternative schedules are evaluated through the combined use of indicators relative to the reduction in demand for irrigation water, the consequent yield reduction and the impacts on farmer's income. The economic evaluation of those strategies is performed with the help of a linear programming model. Results indicate that, when there is limited water availability, each crop reacts differently to the water restrictions applied in terms of yield decrease and gross margin per unit surface and unit of water applied. For average demand, the adoption of deficit irrigation is generally feasible for all crops considered. However, for the tomato crop, there is a trend to restrict the cultivated surface when water scarcity increases. Under high and very high demand conditions, the gross margin per unit of water applied decreases for the potato and the tomato crops but increases for the wheat crop. Then, adopting deficit irrigation is not feasible for the tomato and potato crops, the option being to decrease the cultivated surface. On the contrary, the wheat crop responds wen to deficit irrigation, with a good water valorisation, including under drought conditions. A. Zairi et al.The frequent occurrence of droughts requires pro-active and rational water management approaches that could be helpful to both managers and farmers. Those measures include the reinforcement of the hydraulic infrastructures, increased waste water reuse, the enforcement of restrictions on water volumes supplied for irrigation, and changes in water prices . However, their application require s better knowledge of the climatic variability and respective impact on crop yields.A recent study shows that a decrease of 10 % in water supply to cereal s and fodder crops would result in a reduction of about 2 % in the net agricultural product, mainly as a consequence of a reduction in the irrigated area (DGIRE, 1997). Irrigated wheat surface (60 000 ha) is less than 5 % of the total cereal cropped area in Tunisia but, when drought occurs, the irrigated wheat area contributes with 20 % to the total cereal production, while also playing a main role in seed production (El Amami, 1995).In order to prepare appropriate irrigation strategies for wheat and main field horticultural crops, tomato and potato, when water supply restrictions have to be imposed, the present study was performed to assess how deficit irrigation (English and Raja, 1996) could...

6Irrigated agriculture faces intense competition for water in Mediterranean environments. In 7 this paper, alternate furrow irrigation was explored for a potato crop in the conditions of the 8Cherfech irrigation district, located in the Medjerda project of northern Tunisia. A field 9 experiment was performed involving seven furrow irrigations in three irrigation treatments: 10 alternate furrow irrigation (AFI), fixed furrow irrigation (FFI), and continuous furrow 11 irrigation (CFI). Crop yield and water productivity were determined in all treatments. The 12 experiment involved detailed irrigation evaluation and soil water measurements in the first 13 three irrigation events. Soil infiltration (estimated with a surface irrigation model) was larger 14 for CFI than for AFI or FFI. This finding was confirmed by the average irrigation depths, 15

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Irrigation uniformity and wind drift and evaporation losses (WDEL) are major concerns for the design and management of sprinkler irrigation systems under arid or semi-arid conditions. Field trials were carried out to assess irrigation uniformity and WDEL under various wind velocities, sprinkler spacings and operating pressure heads. Based on experimental data, a frequency analysis was performed to infer the occurrence probability of a given uniformity coefficient (UC). In addition, statistical regressions were used to model WDEL as a function of different climatic variables. Increasing the operating pressure head improved uniformity at low wind speeds. It was shown that UC has been severely impaired at wind speeds above 4 m/s. In the prevailing wind conditions, the frequency analysis showed that a sprinkler spacing of 12 m × 12 m provided the best uniformity. In the local conditions, it is recommended to stop irrigation when wind velocity exceeds 4 m/s. Moreover, it was shown that wind speed and relative humidity were the main significant variables influencing WDEL.