Objective

Estimation of crop water use (evapotranspiration, ET) from weather data (wind speed, air temperature and humidity, and solar irradiance) is widely used for irrigation scheduling. The current paradigm involves estimating crop ET from a daily reference evapotranspiration (ETr), calculated from weather data, multiplied by a crop coefficient, Kc, that is dependent on the growth stage, planting density and species. Knowledge of the relationship between Kc and these variables is lacking for some important crops in China. The objective of this research is to determine Kc versus crop growth stage, planting density and species for important irrigated crops of the region.

Research Approach

Arguably the most accurate method of determining Kc is to measure daily crop water use (ETc) by weighing lysimeter, while simultaneously measuring the pertinent weather variables and calculating a daily value of ETr using the standardized Penman-Monteith reference ET equations. Daily Kc is then computed as Kc = ETc/ETr. The project was to design and build a facility consisting of 24 small weighing lysimeters and one large weighing lysimeter for use at the National Experimental Station for Precision Farming in Xiaotangshan, Beijing, so that crop coefficients could be measured accurately.

Activities

Dr. Xuzhang Xue visited the USDA-ARS laboratory at Bushland, Texas, several years ago to work with Dr. Evett on design of tension infiltrometers for investigations of soil hydraulic conductivity versus soil water potential at different scales. At that time he became familiar with the weighing lysimeters at Bushland used by ARS to determine crop coefficients for use in delivering irrigation scheduling advise to the farmers of the U.S. Southern High Plains in cooperation with Texas AgriLife. In 2007, he discussed the design of a similar facility in China with Dr. Evett, who then began the facility design. The design was completed in 2008 during a visit by Dr. Evett to Beijing, Xiaotangshan and Xian. Construction began in late 2008 and was largely completed in 2009. Future activities will include experiments with crops important to the region to determine crop coefficients under irrigated conditions.