Neutron Moisture Meter Calibration in Six Soils of Uzbekistan Affected by Carbonate Accumulation

Improvements in water use efficiency of the irrigated agriculture of Uzbekistan begin with determination of crop water use under its different climates, soils, and management practices. The neutron moisture meter (NMM) is a key tool for determination of crop water use, which we define here as being equal to transpiration and evaporation from the soil surface, i.e., the evapotranspiration. We accurately field calibrated NMMs at six locations in Uzbekistan, in soils ranging from deep, uniform silt loams of loessial origin to highly stratified alluvial soils near the Amu Darya River. 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 the increased CaCO 3 content in the lower of the two layers. 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 at the 90‐cm depth required separate calibration equations for the 30‐ to 70‐cm depth range (silt loam) and the 110‐ to 170‐cm depth range (fine sand). Overall, the root mean square errors (RMSEs) of calibration ranged from 0.009 to 0.025 m 3 m −3 and r 2 values ranged from 0.91 to 0.99. Data gathered provide an excellent illustration of why calibration efforts should organize soil water content data by depth range. Two examples of profile water content measurement for crop water use studies are given.