A Weighing Lysimeter for Crop Water Use Determination in the Jordan Valley, Jordan

摘要

Efficiency of water use in irrigated agriculture can be improved by providing irrigation scheduling information to farmers. Since 2003, the Middle Eastern Regional Irrigation Management Information Systems (MERIMIS) project has focused on improving irrigation scheduling in Jordan, Palestine, and Israel with cooperators from the region and the U.S. Their efforts have established a network of 15 weather stations to support an irrigation scheduling service using the paradigm that crop water use is equal to a reference evapotranspiration (ET _r ) value (calculated from weather data) multiplied by a crop coefficient. Because crop coefficients developed in one region often do not transfer exactly to another region, crop coefficient values should be developed for those crops commonly grown in the Middle Eastern region and for the agronomic practices, including row spacings, prevalent in the region. Row spacings in this region are often wider than in other regions so that full canopy cover is often not attained, and spacings vary among farmers, leading to an important crop cover factor influence on ET rates throughout the growing season. This article describes the site selection, design, construction, calibration, and preliminary results for a weighing lysimeter built by the MERIMIS team for determination of crop coefficients in the Jordan Valley. Distinct features of the design include the low roof that guarantees at least 1.5 m of soil depth all around the lysimeter, the tall scale-support piers that allow for suspension of the vacuum drainage tanks from the scale so that drainage does not change lysimeter mass, the elevated load cell and datalogger that provide insurance against the unlikely event of flooding, and the rectangular surface that accommodates the wide variety of row spacings used in the Jordan Valley. The lysimeter is sited in the center of a 100 × 200 m drip irrigated field, is 2.4 × 3 m in surface area and 2.5 m deep, and has a calibrated accuracy of 0.11 mm of water and resolution of 0.064 mm. Preliminary data show that the lysimeter and associated weather instrumentation are working as expected and are able to detect half-hourly ET rate responses