|Ayars, James - Jim|
Submitted to: National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: 12/20/2000
Publication Date: N/A
Citation: Interpretive Summary: Decades of irrigation on the west side of the San Joaquin Valley without sufficient drainage have created large areas where shallow groundwater (<1.5 m) has become a problem for agriculture. The presence of toxic trace elements in agricultural drainage water and its effect on the environment have put severe limitations on drainage outflow from agriculture fields and dwater districts. Shallow groundwater causes aeration problems, soil salinization and ultimately yield losses. Because drainage outflow is restricted and may need to be reduced even further, irrigation reduction is a farm management solution for this situation. One possibility to reduce the amount of applied irrigation water is to include the potential shallow groundwater use when scheduling irrigation. It has been shown that several crops have the potential to use saline groundwater for crop water use. Reducing the amount of applied irrigation and increased crop water use from mgroundwater will result in lower drainage outflows. Potential groundwater use by plants depends on depth to the groundwater and groundwater salinity. The higher the salinity level of the groundwater and the deeper it is, the less the potential for shallow groundwater use. Crop coefficients were developed to adjust irrigation management in the presence of a shallow water table based on the depth to the groundwater and groundwater quality. A base coefficient was used to represent the crop water use without a groundwater table present. Crops can use groundwater with higher salinities than indicated by the Maas-Hoffman salt tolerance thresholds. Objectives for this study were to develop a base crop coefficient for the short season cotton variety (Paymaster 1680 Bollguard), and to study soil water uptake processes in the presence of saline shallow groundwater.
Technical Abstract: Two weighing lysimeters were used to measure crop evapotranspiration of subsurface drip irrigated cotton. A groundwater table was maintained in one of the lysimeters. Groundwater use as part of crop evapotranspiration was characterized using a Mariotte Bottle and hourly measurements. Lysimeter evapotranspiration data were compared with sap flow measurements. .Irrigation management, changes in crop water requirements, and changes in soil hydraulic conductivity were shown to affect saline groundwater use. Groundwater contribution of up to 30% of daily crop water use was measured. Data were compared to previously developed adjusted crop coefficients, and some adjustments to these coefficients were made. The largest groundwater contribution was shown to occur at the end of the growing season, when roots are fully developed and irrigation is terminated. Differences in root development were observed in the presence of shallow groundwater compared with no groundwater table.