Dissolved-Solids Loads Discharged from Irrigated Areas near Manila, Utah, May 2007–October 2012, and Relation of Loads to Selected Variables

Abstract

The Manila/Washam Salinity Project (MWSP) is a cooperative effort by the Natural Resources Conservation Service (NRCS) and local farmers and ranchers to reduce the transport of dissolved solids to Flaming Gorge Reservoir from irrigated agricultural lands near Manila, Utah. To estimate dissolved-solids loads from the MWSP area, discharge and water quality from Birch Spring Draw and other selected outflows and inflows were monitored from May 2007 to October 2012. An average annual May–April streamflow of 5,960 acre-feet discharged from Birch Spring Draw at site BSD-2 to Flaming Gorge Reservoir during 2007–12, containing an average dissolved-solids load of 14,660 tons. An average May–April net dissolved-solids load of 24,300 tons per year discharged from the MWSP area, estimated from the relation between streamflow and dissolved-solids concentration at site BSD-2 and other measured inflows and outflows.

The amount of precipitation and water available for irrigation are important factors affecting the dissolved-solids load in outflow from the MWSP area. Net dissolved-solids load discharged from the MWSP area increased with increasing canal streamflow and precipitation measured at Manila during the irrigation season, from May to October, each year. The net tons of dissolved solids discharged from the MWSP area per acre-foot of canal water increased with increasing irrigation season precipitation during May 2007–October 2012.

Irrigation improvements began to be implemented in 2007 to reduce dissolved-solids loads discharged from the MWSP area. The theoretical annual net dissolved-solids load where the cumulative NRCS calculated dissolved-solids load reduction is added to the net MWSP dissolved-solids load is what would be expected if there was no irrigation improvement in the area associated with the MWSP. The theoretical data points lie very near the baseline representing the pre-MWSP dissolved-solids load to canal streamflow relation. The proximity of the theoretical data points to the baseline shows that the NRCS calculations of reduction in dissolved-solids load are generally supported by the data collected during this study.

First posted April 16, 2015