MEASURING GROUNDWATER RECHARGE WITH LYSIMETRY AND THE UNSATURATED ZONE WATER BALANCE BENEATH A FLOOD IRRIGATED CORN FIELD

Groundwater recharge beneath land surface applications of water is an important component of the hydrologic cycle in the State of Colorado, USA. The majority of land applications are for irrigation and stream flow augmentation. For example, the South Platte River flows year-around due to subsurface irrigation return flows. The majority of water used for irrigation comes from surface water, so these return flows are vital for the users downstream to meet their irrigation needs. The water rights laws of Colorado treat all irrigation water applied in excess of consumptive use to be recharging groundwater. Therefore, if the amount of water applied is diminished, the historic return flows for that user must be met in some other manner (e.g., recharge ponds).

The purpose of this study is to measure the amount of deep percolation of flood irrigation water beneath a corn field using lysimetry and the unsaturated zone water balance (UZWB). Few studies of this kind are performed under flood irrigation due to the various amounts of applied water along the length of the field and the potential effects of heterogeneity on infiltration rates. The field has been divided into three blocks. One block is fully irrigated while the other two blocks are under deficit irrigation, with differing amounts of water applied and application rates. Each field has two drainage lysimeters installed beneath the root zone at one-quarter and three-quarter lengths of the field. Neutron probe access tubes are installed to 7 m depth at one-half the field length in addition to the locations above. The data collected from the 2011 and 2012 growing seasons will be discussed. For the 2011 season the two methods compare favorably at all but a few locations. HYDRUS 1-D modeling of unsaturated flow through the root zone suggests that these discrepancies are due to localized layering in the field. Our results show that the UZWB method is a minimally intrusive, viable method for monitoring groundwater recharge under flood irrigation conditions and to estimate the volume of water the user needs to discharge into recharge ponds if land application rates are changed.