Quantifying drought for humid, temperate pastures using the Crop Water Stress Index (CWSI)

Pastures benefit from changes in management during periods of drought; therefore, better methods of quantifying drought would be useful. This research was conducted to determine how values of the Crop Water Stress Index (CWSI) related to measured reductions in evapotranspiration (ET) by pasture as a result of water stress. Evapotranspiration by cocksfoot was measured using a large weighing lysimeter at Coshocton, Ohio, during 1988, a very dry year, and 1989, a wet one. Evapotranspiration by cocksfoot and tall fescue was measured during 1993, at Kearneysville, West Virginia, using two large weighing lysimeters with drought simulated during the latter part of the growing season using rain-out shelters. Net radiation, wind speed, air temperature, humidity, and canopy temperature were measured at both locations to calculate potential ET and the CWSI. The relationship between CWSI and relative ET (actual divided by potential ET) was curvilinear and differed between cocksfoot and tall fescue. Under very dry conditions, the CWSI exceeded values which have been reported as the maximum for field crops. While the CWSI has the potential to be a useful tool for quantifying pasture water stress, there is a species dependency that will require further analysis.