Appropriate irrigation scheduling determination of major crops and orchards in Honam, Lorestan traditional irrigation network

Agricultural water usage efficiency is one of the important goals of water productivity improvement program of Jahad-e Agriculture Ministry in Iran. In this case, the determination of appropriate irrigation scheduling, which is consisted of irrigation depth and frequency, is a key strategy. Many of the country’s agricultural plains are under traditional irrigation scheme and unlike modern irrigation networks have a more complex consumption pattern. This study was carried out to determine the irrigation depth and scheduling of major orchards of Honam sub-catchment of Karkheh River Basin (KRB), in order to optimize agricultural water consumption at basin level. Honam sub-catchment was selected as the research pilots of KRB. Nineteen irrigation homogenous units supplied from joint water resources were delineated using ArcGIS 10.3 to predict irrigation depth and frequency of cropping pattern in 913.8 ha irrigated lands of study area. In addition, the land use map of crops and orchards in the irrigated lands was prepared by field survey and combining satellite images of irrigation channel, cultivation type and their distribution. The 15-years (2000 – 2014) data of Alashtar weather station were collected and the reference evapotranspiration (ET0) was calculated based on Penman-Montith method using ET Calculator software. Moreover, the length of plant growth period of cultivation pattern was determined by completing a questionnaire. Then, evapotranspiration of cropping pattern (ETc) was calculated considering proposed Kc of FAO-56. Effective rainfall was deducted from ETc and finally, irrigation requirement of crops and orchards were calculated by subtraction of ETc and effective rain as ten-day, monthly and entire growth period. The related soil physical properties for irrigation purposes including soil texture, Field Capacity (FC), Permanent Wilting Point (PWP) and water holding capacity, were measured at different soil depth at each irrigation homogenous units. The net irrigation requirement of winter wheat in Honam sub-basin was calculated to be 318.1 mm. The maximum amount of net irrigation required for wheat was 112 and 5.7 mm in May and March, respectively. The annual and monthly maximum and minimum irrigation requirements for sugar beet were 842.3, 204.4 and 19 mm, respectively. For mixed orchards, the annual irrigation requirement was 993.4 mm, and the monthly maximum and minimum irrigation requirements were 230.2 and 14.8 mm, in August and April, respectively. For alfalfa, the annual irrigation requirement was 1190.7 mm, and the monthly maximum and minimum irrigation requirements were 284.4 and 7 mm, in July and March, respectively. Among the crops, the maximum irrigation requirement was related to alfalfa and then mixed orchard, and the minimum irrigation requirement was related to winter wheat with 318.1 mm during the growing season. The results also showed that the most of the irrigation homogenous units soil texture were silty clay loam and clay. Due to the favorable rainfall in autumn, winter and April, the plant's water requirements are met by rainfall. Therefore, no irrigation is required except in May and June. Wheat irrigation period in May was calculated as 16 days on average among all water units, and 11 days in June. In June, when the maximum water requirement of wheat occurs, the maximum irrigation interval for wheat is 13 days in all-homogenous units of 4 and 14 and the minimum irrigation interval is 10 days in all-homogenous units of 6, 10, 12 and 16. The irrigation interval for sugar beet in May was 21 days on average among the same homogenous units and in June. The irrigation intervals for sugar beet in July, August, September and October were 11, 7, 8 and 11 days, respectively. In July, when the maximum water requirement of sugar beet occurs, the maximum irrigation interval was 9 days in homogenous irrigation units of 14 and 15, and the minimum irrigation interval was obtained 7 days in irrigation units of 6, 10, 12 and 16. The irrigation periods for orchards and horticultures among all units in June, July, August, September and October were 17, 10, 10, 12 and 19 days on average, respectively. Alfalfa irrigation interval is shown in the units of 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 17, 18, and 19 of irrigated lands of Honam sub-catchment. Meanwhile, its irrigation intervals among all irrigated units in April, May, June, July, August and September were 29, 21, 10, 7, 7 and 10 days on average, respectively.