THE EFFECTS OF FLOOD IRRIGATION VS. PIVOT IRRIGATION ON GROUNDWATER RECHARGE

Groundwater can be an important resource for river flow support, and this is especially true with the Big Hole River in southwestern Montana which is mainly fed by groundwater late in the summer season. Maintaining the source of groundwater is an important focus both for the Big Hole River Watershed Committee as well as local ranchers. Agriculture uses large quantities of water, and as technology advances many ranchers are turning to the use of pivot lines for irrigation. Historically, flood irrigation was used when surface water was available. Surface water sources include tributary streams as well as diversions from the main channel of a river. Flood irrigation is considered less efficient when compared to pivot line irrigation as a fraction of the applied water is ‘lost’ as infiltration to groundwater. The Melrose Valley primarily utilized flood irrigation methods until pivot line irrigation was introduced. Slowly, the valley farmers and ranchers have transitioned to pivot line. In this study three lysimeters were installed, one in a pivot line irrigated field sowing alfalfa, one in a flood irrigation field sowing grass hay, and the third in an area with no crop or irrigation. The purpose was to quantify how much water from each method passed through the root and vadose zone of the soil profile to potentially become groundwater recharge. Each field site was studied through one growing season. A model for each was made utilizing the program HYDRUS. The models were used to gain an understanding of how surface water may be moving through the soil profile for both fields. Only the flood irrigated field showed any flux, or positive groundwater recharge. Total flux after the active irrigation period reached 62,000 cubic centimeters. Total recharge calculated for the flood field was 28.8 million liters. The pivot irrigated field showed no flux, and so there were no model results to show. The results are an important part of understanding what role irrigation plays in groundwater recharge as well as how surface water travels through the soil profile to become recharge.