Large weighing lysimeters for developing evapotranspiration crop coefficients and efficient irrigation strategies for high-value vegetable crops

A large, 25-ton weighing lysimeter was installed in 2003 to measure daily water use in vegetable crops grown in central California.  An identical lysimeter was also installed in an adjacent grass field to measure potential evapotranspiration (ET).  Since then, the vegetable lysimeter has been planted with several crops, including broccoli, bell pepper, head lettuce, and garlic.  Information on seasonal water use is now available for these crops.  In this presentation, the utility of crop coefficients (determined as the ratio of crop ET to potential ET) will be discussed along with the methodology used to develop such crop coefficients for vegetable crops.  Included in the discussion is the relation of canopy cover to crop water use.  Such relationships enable weather-based irrigation scheduling based on relatively simple estimates of canopy cover.  Information on irrigation trials from the surrounding lysimeter field will also be presented to identify the optimum method and level of irrigation for each vegetable crop.  Irrigation in these trials was automatically controlled using the weighing lysimeter.  In most cases, irrigation use was optimized for maximum yield when irrigation was scheduled at 75-100% of the estimated crop ET but differed little between plants irrigated by drip or subsurface drip.  Data will also be presented from crops irrigated using saline water.  Procedures will be discussed on how to adjust crop coefficients for water limitations and salt stress.