Soil Water Balance and Shallow Aquifer Recharge in an Irrigated Pasture Field with Clay Soils in the Willamette Valley, Oregon, USA

Abstract

Quantifying soil water budget components, and characterizing groundwater recharge from irrigation seepage, is important for effective water resources management. This is particularly true in agricultural fields overlying shallow aquifers, like those found in the Willamette Valley in western Oregon, USA. The objectives of this two-year study were to (1) determine deep percolation in an irrigated pasture field with clay soils, and (2) assess shallow aquifer recharge during the irrigation season. Soil water and groundwater levels were measured at four monitoring stations distributed across the experimental field. A water balance approach was used to quantify the portioning of different water budget components, including deep percolation. On average for the four monitoring stations, total irrigation applied was 249 mm in 2020 and 381 mm in 2021. Mean crop-evapotranspiration accounted for 18% of the total irrigation applied in 2020, and 26% in 2021. The fraction of deep percolation to irrigation was 28% in 2020 and 29% in 2021. The Water Table Fluctuation Method (WTFM) was used to calculate shallow aquifer recharge in response to deep percolation inputs. Mean aquifer recharge was 132 mm in 2020 and 290 mm in 2021. Antecedent soil water content was an important factor influencing deep percolation. Study results provided essential information to better understand the mechanisms of water transport through the vadose zone and into shallow aquifers in agricultural fields with fine-textured soils in the Pacific Northwest region in the USA.