Irrigation Scheduling As Affected By Field Capacity and Wilting Point Water Content from Different Data Sources.

Soil water content at field capacity and wilting point water content is critical information for irrigation scheduling, regardless of soil water sensor-based method (SM) or evapotranspiration (ET)-based method. Both methods require knowledge on site-specific and soil-specific Management Allowable Depletion (MAD) at certain depth. MAD is commonly defined as certain percentage (i.e., 50% widely used) of plant available water content (AWC, difference between field capacity and wilting point water content). Soil water storage in rooting zone either measured by soil sensors or estimated by initial soil water moisture and ET is compared with MAD. Once SW is equal or less than MAD, an irrigation is trigged and soil water profile was recharged to any percentage of its field capacity. Therefore, field capacity and wilting point water content can affect both irrigation timing and amount. Selection or measurement of MAD can greatly affect when and how much an irrigation should be applied. Most irrigation scheduling tools obtain field capacity and wilting point water content data from either NRCS SSURGO database or a soil texture based database/pedotransfer functions. We tried to utilize SSURGO database for irrigation management. Soil physical and hydraulic properties in SSURGO database were evaluated at a field scale in the west Delta and east Blackland Prairie of Mississippi. Forty soil samples from top 0-15 cm at grid cell of 50 x 50 m were taken in a 7 ha field located in Delta. Difference in clay content between measured and SSURGO values ranged from 19 to 74%, while difference in silt content ranged from 30 to 55% for 80% of soil samples. Soil samples of three soil types, Vaiden silt clay, Okolona silt clay and Demopolis loam, were taken in Blackland Prairie. AWC was measured in comparison with SSURGO and soil texture based values for irrigation scheduling. Since both Vaiden and Demopolis are silt clay, their soil texture based AWC is 0.12, while Demopolis loam is 0.16. Only a range of AWC is provided in SSURGO database which is 0.1-0.15 for Vaiden soil all the way down to 100 cm depth, 0.2-0.22 at 0-18 cm and 0.18-0.2 at 18-100 cm depth for Okolona soil, 0.1-0.17 at 0-23 cm and 0.03-0.06 at 23-100 cm depth for Demopolis soil. In contrast, measured AWC value is 0.23 at 0-15 cm and 0.21 at 15-100 cm depth for Vaiden soil, 0.14 at 0-18 cm and 0.11 at 18-100 cm depth for Okolona soil, 0.18 at 0-23 cm and 0.14 at 23-100 cm depth for Demopolis soil. As an example of Demopolis soil, on-site measured AWC could trigger an irrigation 10 more days earlier than AWC from other data sources for soybean in Mid or late July, 2014. 20 mm difference in irrigation amount for replenishing soil profile was found among those trigger criteria. In comparison of measured values to the commonly used soil textured based AWC, 13 days difference in irrigation timing and 60-70 mm difference in maximum root zone was calculated. Compared measured AWC with SSURGO values, differences of 18 days in irrigation timing and 60 mm in irrigation amount were calculated. It suggests that field capacity and wilting point water content as critical values for determining irrigation timing and amount should be measured on-site, soil texture based AWC and SSURGO database values are not always reliable for irrigation scheduling at field scale.