Abstract
As agricultural water resources in Asia become increasingly scarce, the irrigation efficiency of rice must be improved. However, in this region there is very limited information available about water use efficiency across spatial levels in irrigation systems. This study quantifies the volume of water reuse and its related cost-benefits at five different spatial levels, ranging from 1500
ha to 18,000
ha, under gravity-fed irrigation system in Upper Pumpanga River Integrated Irrigation System (UPRIIS), Philippines. The major sources of water reuse are considered, namely groundwater pumping, pumping from creeks, combined use and irrigation supplies from check dams. The volume of water available from all four sources of water reuse was quantified through extensive measurements. Production functions were developed to quantify water-yield relationships and to measure the economic value of water reuse. This study was conducted during the dry season of 2001, which existed from 19 November 2000 until 18 May 2001.
The water reuse by pumping and check dams was 7% and 22% of the applied surface water at District 1 level. The reuse of surface water through check dams increased linearly with 4.6
Mm
3 per added 1000
ha. Similarly, the total amount of reused water from pumping is equivalent to 30% of the water lost through rice evapotranspiration during the dry season 2001. The results showed that water reuse plays a dominant role in growing a rice crop during the dry season.
The result showed no difference in pumping costs between the creek (US$0.011/m
3) and shallow pumps (US$0.012/m
3). The marginal value of productivity (MVP) of water reuse from creek (US$0.044/m
3) was slightly higher than the water reuse through the pumping ground water (US$0.039/m
3). Results also indicated that the total volume pumped per ha (m
3/ha) was ranging from 0.39 to 6.93
m
3/ha during the dry season.
The results clearly indicate that the quantification of amount of water reuse is very crucial for understanding and finding of water use efficiency at the irrigation system level. The results also revealed that rice production systems are still profitable despite high pumping costs and other associated expenses at all spatial levels in District 1. More than 1500 farmers, from a total of 10,000, use 1154 pumps to draw water from shallow tube wells (or from drains and creeks) for supplementary irrigation at a District level. Reuse of water plays a vital role in growing a profitable rice crop during the dry season.