A computer model for design of microcatchment water harvesting systems for rain-fed vineyard

This paper describes a simple simulation model for design of microcatchment water harvesting systems for rainfed vineyard in Bajgah area, Fars province, Islamic Republic of Iran. The model includes rainfall, runoff, actual evapotranspiration, water contribution from deeper layers and deep percolation. In this study, 10 microcatchments containing grape tree with area of 13.4 m2 were built in Bajgah area and volumetric soil water contents at depth of 120 cm and amount of soil water at depths of 0–120 cm were measured during the growing season of 1984–1987 in these microcatchments. In this simple model, daily rainfall, runoff and daily actual evapotranspiration in Bajgah area during years of 1984–1987 were used, and the volumetric soil water contents in each day of year were calculated. The values of water contribution from deeper soil layers were 0.14 mm per day for the first to fifth layers and 0.01 mm per day for the sixth and seventh layers (a total of 0.72 mm per day). The results showed that, by using Penman–FAO formula there was no significant difference between the values of predicted and measured volumetric soil water contents in depth of 120 cm, and amount of soil water at depths of 0–120 cm. On the other hand, determination of the microcatchment area depends on active root depth, soil water holding capacity, runoff coefficient, cultivated area, annual rainfall and actual evapotranspiration. The comparison between total measured yield with tree microcatchment area of 13.4 m2 and total estimated yield with tree microcatchment area according to the annual rainfall of the same year, showed that there was no significant difference between them. The microcatchment area according to the amount of annual rainfall with 90% probability of occurrence was estimated to be 21 m2, but with lower probability of occurrence (higher value of annual rainfall), microcatchment area decreased. Meanwhile, with decreasing microcatchment area, although the yield of each tree decreases, but the number of trees in a unit area (ha) increases. Therefore, the total yield in unit area (ha) will increase. So, using smaller microcatchment area is more appropriate. Therefore, due to the spacing of 3 m×3 m for grape plantation in Bajgah area, the best microcatchment area is 9 m2.