Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2001
Publication Date: 10/1/2001
Interpretive Summary: In 1997, Oklahoma winter wheat production covered 5.4 million acres or 58% of the total harvested acreage and 12% of the total state area (Oklahoma Agricultural Statistics Service, 1998). As elsewhere, the threat of groundwater contamination by nitrates from fertilizer has been a concern for many years. To estimate the threat of groundwater contamination by nitrates requires that the rate of water flow towards the groundwater be known. Few measurements of water flow beneath the root zone are available for winter wheat fields in Oklahoma. The objective of this study was to estimate the water flow rate beneath the root zone under experimental dryland winter wheat plots in Northern Oklahoma near Stillwater. The average annual precipitation for the geographic area under consideration was about 34 inches. The water flow rate was measured indirectly by use of a chemical tracer. The estimated water flow rates under dryland winter wheat ranged from 0.5 to 1.5 inches per year and increased with increasing nitrogen fertilizer application. A possible explanation for the increased water flow rates under fields with higher nitrogen application is the more vigorous plants and root systems produced a soil structure with greater infiltration and less surface runoff. The findings of this study will be used in subsequent studies assessing the movement of nitrates in the soil and the threat of groundwater contamination by nitrates.
Technical Abstract: The utilization of Chloride Mass Balance (CMB) to determine water fluxes has generally been restricted to applications in arid to semi-arid environments, because only in such environments does the chloride deposited by precipitation and dry fallout concentrate sufficiently by evapotranspiration for accurate measurement. This study successfully applied CMB to dryland winter wheat plots with 860 mm of precipitation per year. Soil cores were collected from long-term dryland winter wheat test plots located near Stillwater, OK, which had known, constant applications of the fertilizer KCl for the past 29 years. This additional chloride was sufficient to allow for accurate chloride concentration measurement. Groundwater recharge rates of 12.2 to 38.9 mm/yr were calculated with recharge increasing with fertilizer N. These fluxes may be overestimated by up to 20% based on anion exclusion measurements from adjacent soil cores. Numerical modeling of the chloride distributions beneath the plots supported the assumptions of CMB.