Estimating Drainage under Cotton with Chloride Mass Balance and an EM38

Methodologies that use electromagnetic induction and resistivity in soil profiles to estimate drainage across whole fields and catchments require complex models or measurement systems that are not easily available or difficult to apply at the farm level. The objective of the present study was to define a methodology that could use information easily available to agronomists and commercial cotton consultants to estimate drainage in irrigated Vertisols using a chloride (Cl-) mass balance approach. A secondary objective of this study was to eliminate or minimize the expensive and tedious laboratory analyses for determining Cl- concentration in soils. A model was developed using electromagnetic induction measurements taken with an EM38 instrument in the horizontal mode (EMH) to estimate the chloride concentration in a saturated soil extract in the 0- to 1.2-m depth of irrigated Vertisols. A stepwise linear regression model where the independent variables were soil water storage, exchangeable sodium percentage, and EMH predicted chloride concentration the best. The chloride concentrations thus estimated were similar to measured values only when measured chloride concentrations in the saturated extract were 10 meq L-1 (355 mg L-1 = 240 mg kg(-1)). These values of chloride were then inserted into a chloride mass balance model to estimate deep drainage. In more saline soils, large differences occurred between measured and estimated chloride. Values of drainage based on estimated chloride concentrations in the range of 0-10 meq L-1 were very similar to those based on measured chloride concentrations. There is potential to use an EM38 for quick assessment of deep drainage under irrigated conditions in the field.