Re-use and cyclic use of water saving in rice cultivation in gravity irrigation system of Philippines

Re-use and cyclic use of water saving in rice cultivation in gravity irrigation system of Philippines

This paper presents the situation analysis of a re-use system of return flow for irrigation in District 1 of the Upper Pumpanga River Integrated Irrigation System (UPRIIS), Philippines. While elaborating the situation analysis, emphasis is given to quantification of water re-use and its related economic benefits under gravity-fed irrigation systems. This irrigation system presents a distinct geophysical feature which provides opportunities for capturing irrigation outflows from rice fields of upstream areas through a network of natural creeks. These creeks feed into check dams which divert irrigation supplies for downstream areas. In UPRIIS area, the major sources of re-use system include groundwater pumping, lifting surface water from creeks and irrigation supplies from check dams.

Specifically, this study aimed to: a) quantify the current level of water re-use from creeks, groundwater and check dams; and b) to estimate the economic benefits of water re-use from pumping groundwater and creeks in District 1 of UPRIIS.

This study was conducted during the dry season of 2001, (19 November 2000 - 18 May 2001). During this period, the average rainfall was only 190mm (long term average of 170 mm). A comprehensive survey was carried out in District 1 with the help of the National Irrigation Administration (NIA). This survey focused on obtaining information regarding pumping technologies, sources of re-use water (groundwater or creek), existing cropping patterns and pumped water sharing practices. An additional 50 pump owners, from a total of 1154 pumps, were selected for detailed investigation and monitoring of total water with drawl from different sources of water.

The geographic information system (GIS) was developed to characterize the study area. To quantify the amount of water re-use, pumps were calibrated 7-9 times with a V-notch weir to determine the actual discharge for different sizes of pump. The total amount of water being pumped by each monitored farm was determined by multiplying the actual discharge with the total pumping times/hectare. Water and yield relations were determined by developing production functions, and economic benefits were determined by estimating the water productivity (economics) and calculating the marginal value of water.

In the study area, it was found that 22% of the farmers use pumps to draw water from shallow tube wells for supplementary irrigation. The pump density ranged from 0.13-0.2 pumps/ha, showing that the majority of the farmers depend on groundwater. The estimated total volume pumped per ha ranged from 0.39-6.93 m3/ha during the dry season. Overall, this is equivalent to 30% of the water lost through seasonal actual evapotranspiration from the rice crop. There was no difference in pumping cost between the creek (0.012 $/m3) and shallow pumps (0.012 $/m3). Overall marginal value product (MVP) of water re-use was 0.0406 $/m3. The MVP of water re-use from creek (0.0438 $/m3) was slightly higher than the water re-use through the pumping ground water (0.0388 $/m3).

The results showed that water re-use plays a dominant role in the growth of a rice crop during the dry season. The results clearly indicate that the quantification of volumes of water re-use is crucial for understanding and finding real water saving possibilities at the irrigation system level. The results also revealed that rice production systems are still profitable despite high pumping costs and other associated expenses in District 1. These findings would lead to an improvement in the water use efficiency and water productivity of irrigated rice systems.