Water Availability for irigation in Kayangan Watershed, Yogyakarta, Indonesia

This research was located in Kayangan Watershed, Yogyakarta Province, Indonesia. The aims of this research are to evaluate the water balance for irrigation and to predict the correct cropping pattern. Water Balance for irrigations computed using computations of the available water and irrigation water requirements. Watershed’s available water is computed using Thornthwaite-Mather’s method. Irrigation water requirements are computed using the equations of Crop Water Requirements (CWR), Farm Water Requirements (FWR) and Project Water Requirements (PWR). The cropping pattern which has been practiced until now is rice plant for 1^st and 2^nd period and soybean in the 3^rd period (paddy-paddy-soybean). This cropping pattern will be adjusted if the irrigation water requirements are not suitable to the available water.

The calculation shows that the available discharge water are vary between 30 l/s up to 2570 l/s. The biggest discharge was occurred in February, and the smallest discharge was occurred in October. Conjunctions between available water and crops water requirements indicates water deficiency during the 1^st paddy period during October, with the lack of amount are 170 l/s up to 174 l/s. The entire soybeans cropping period also indicates deficiency with the lack of amount between 16,5 l/s up to 28,3 l/s. Water deficiency could be decreased by intermittent irrigation flow, sprinkle and by substitute soybean with corn. Corn doesn’t demand much water and only need to be sprinkled 4 times during dry seasons. Corn fill in the new cropping pattern, paddy-paddy-soybean.