ABSTRACT: In a 16‐year paired watershed study in Arizona, a 303‐acre chaparral watershed was treated by helicopter in a mosaic pattern (55 percent of watershed) with tebuthiuron herbicide pellets to control the moderately dense, heavy water‐using chaparral. The objective was to determine if annual stream discharge could be increased without degrading other resource values. An adjacent untreated chaparral watershed served as the control on which to base the water quantity and quality treatment effects. Annual streamflow was increased by 1.5 to 5 inches over a seven‐year evaluation period. The streamflow increase was accompanied by a small but statistically significant increase in nitrate concentration. The relatively small nitrate response was attributed to the mosaic treatment. Nitrate released from the converted areas was diluted by streamflow from untreated areas, thereby reducing nitrate concentrations in streamflow at the watershed outlet.
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