Submitted to: ASAE Annual International Meeting
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2001
Publication Date: N/A
Citation: Interpretive Summary: Disposal of shallow groundwater collected by subsurface drainage systems used in agricultural production is one of the major water quality problems facing irrigated agriculture. Source control and reuse have been identified as two options for reducing the volume of agricultural drainage. In the first instance, the volume of drainage is reduced through improved irrigation water management resulting in less deep percolation. In the second case, the volume is reduced by using the water for supplemental irrigation. An alternative to using drainage water for irrigation is to manage the shallow groundwater to induce in-situ use by crops. Restricting drainage flow modifies the system hydrology and changes the timing of the peak drainage discharge. Field studies in 2000 on the westside of the San Joaquin Valley in California where drains were restricted show that the effective depth of the drains was raised 210 cm at installation to an average depth of 177 cm. Changes in hydrology due to restricting drainage flow resulted in smaller fluctuations in drainage water flow rate. When the rootzone of the crop is not well developed, a larger percentage of irrigation water is drained than later in the season. This is a result of a larger available pore volume in the vadose zone for irrigation water storage. Irrigation scheduling was not modified in this study. It is likely that with a shallower groundwater depth due to restricted drain flow, the irrigation schedule could be adjusted, using longer periods between irrigation applications and smaller irrigation applications, possibly resulting in reduction of the number of applications. An integrated approach to irrigation and drainage management is recommended, compared to only drainage management.
Technical Abstract: Drainage volume and irrigation applications were measured for 2 quarter sections on the west side of the San Joaquin Valley, CA for a three year period. Baseline flow data were collected the first year of the project (1998). In subsequent years, the groundwater level in quarter sections growing cotton was maintained at 1.2 m below field level through restricting the drainage outflow from artificial subsurface drains at the field outlet. The groundwater level in quarter sections growing crops other than cotton was left at the depth of the drains (2.1 m). Data collected on flow are compared and analyzed on accuracy. Drainage flow response is analyzed for a field with comparable crop and irrigation management in the baseline year as in 2000. Although the hydrology of the tile drain system and groundwater appears to have changed, the reduction in drainage water volume was small. Restricting drainage flow without adjusting the irrigation scheduling is less effective than an integrated approach to irrigation and drainage management.