|Evett, Steven - Steve|
Submitted to: World Congress of Soil Science
Publication Type: Proceedings
Publication Acceptance Date: 2/11/2002
Publication Date: 8/1/2002
Interpretive Summary: The soil moisture neutron probe (SMNP) is used to measure soil water content at different depths in the crop root zone and below. Such measurements are key for determination of crop water use and for irrigation management. The SMNP must be calibrated for specific soils. Methods and equipment for accurate calibration, developed by ARS at Bushland, Texas, have been used successfully on several Great Plains soils. We used the sam methods and equipment to calibrate the SMNP on five important irrigated soils of Uzbekistan. Results showed that accurate and useful calibrations were obtained; and that soil layers, similar in calcium carbonate content to those found in the Great Plains, caused similar differences in calibration parameters. This exercise validated the methods and equipment developed at Bushland on soils in a very different setting and provided methods for scientific irrigation management in Uzbekistan. These methods will be useful in reducing over-irrigation and salinization, which are important problems in water-poor central Asia, as they are in irrigated areas of the arid and semi-arid western United States.
Technical Abstract: The soil moisture neutron probe (SMNP) is a key tool in measurements of crop water use, necessary for accurate irrigation and minimization of salinization; but it is not useful in all soils. We showed that the SMNP could be accurately field calibrated at five locations in Uzbekistan, in soils ranging from uniform silt loams of loessal origin to highly stratified alluvial soils. All calibrations used four access tubes, two in a dry soil site and two in an adjacent site that was wetted to field capacity. Thus, a wide range of water contents could be sampled at each depth in the soil. This allowed us to discern whether different calibrations pertained to different soil layers or horizons. In all soils, separate calibrations were found for the 10-cm depth due to closeness to the soil-air interface. Near Tashkent and at the Syrdarya Branch Station, the soil below 10 cm was divided into two layers based on increased CaCO3 and/or CaSO4 content of the lower of the two layers. Similar to results in the Texas Panhandle, distinctly different calibration equation slopes were found for these layers. At the Kashkadarya Branch Station, a single calibration was sufficient for the soil below 10 cm. At the Khorezm Branch Station, an abrupt change in soil texture near 70 cm depth caused separate calibration equations for the 30 to 70-cm depth range (silt loam) and the 110 to 170-cm depth range (fine sand). Calibration at the Fergana Branch Station was successful for the 10-cm and the 30 to 90-cm depth ranges. Root mean squared errors (RMSE) of calibration were in the range of 0.009 to 0.025 m**3/m**3 and r**2 values ranged from 0.91 to 0.99. Two examples of profile water content measurement for crop water use studies in 2001 are given, one for winter wheat and one for cotton.