Graduate Thesis Or Dissertation

Surface Water - Groundwater Interactions in Clayey Pasture fields in the WIllamette Valley

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  • As the western United States continues to experience prolonged drought that is extending water deficits and threatening ecosystem resilience and socioeconomic systems, it will be vital to understand the relationship between water use and transport for proper water resource management. This is especially important to the agricultural areas of the Pacific Northwest, which are facing substantial pressures in water management and allocation between agriculture use, hydropower, aquatic ecosystem requirements, and fighting wildfires. Agricultural lands in the Willamette Valley rely on precipitation and surface water irrigation as the main water inputs making them a large component of the regional water balance and soil water cycles. As a result, it is vital for water managers and farmers to gain a better understanding of surface water and groundwater (SW-GW) interactions to ensure proper quantification of field water budget components, aquifer recharge, and pollutant leaching (e.g., nitrogen) to improve overall water management decisions impacting irrigation demand, groundwater return flows, and aquifer recharge. This investigation took place in a 2.1 ha livestock grazed irrigated pasture field (44.568 Lat.; -123.301 Long.) and a 2.9 ha non-irrigated pasture field (44.567 Lat.; -123.306 Long.), referred to as IRR_FLD and N_IRR_FLD respectively, located at the Oregon State University (OSU) Dairy Center in Corvallis, Oregon, USA. Soil texture analysis revealed that both fields have high clay content ranging from 21.4% to 44.9% in IRR_FLD and 18% to 51.5% in N_IRR_FLD. This investigation consists of two chapters to explore the specific connections between SW-GW as they relate to water transport through the vadose zone into the shallow aquifer and soil moisture dynamics for IRR_FLD and N_IRR_FLD. Chapter one analyzed soil water budget components and characterized groundwater recharge from irrigation seepage using the soil water balance method (SWBM) and the water table fluctuation method (WTFM) in IRR_FLD. Four soil monitoring stations equipped with soil sensors at 0.2 m, 0.5 m, and 0.8 m depths and a monitoring well were used to calculate field water budget components (in mm), including irrigation applied (IRR), precipitation (P), soil water storage (S), actual evapotranspiration (AET), deep percolation (DP), and aquifer recharge (Re). This two-year study (2020 and 2021) took place during the irrigation seasons lasting from 27 July to 12 September 2020 and from 15 June to 9 September 2021. Results showed that DP was variable between stations and irrigation seasons. At the end of the 2020 irrigation season, the South station had the highest aggregate DP (98 mm), followed by the West (94 mm), East (69 mm), and North (20 mm) stations. In 2021, DP was highest in the West station (153 mm), followed by the East (101 mm), South (99 mm), and North stations (92 mm). The amount of DP did not appear to be related to total water applied (TWA = IRR + P) as the largest water inputs did not necessary result in the highest amount of DP. For example, in 2020, the East station had the largest cumulative TWA (280 mm) but the season's lowest DP (69 mm). Furthermore, a linear regression analysis showed that relationships between TWA and DP were low (p > 0.05) for all stations, but antecedent soil water content (TWA - S) was an important factor for calculating DP (p < 0.05; R^2 values ranged from 0.72 to 0.90). IRR events led to sharp rises in groundwater levels which subsided until the following IRR event. Overall, groundwater level remained relatively high at all stations throughout both seasons. Total Re in 2020 ranged from 128 mm in the North station to 137 mm in the West and East wells. In 2021 total Re ranged from 190 mm in the West to 352 mm in the East well. Kruskal Wallis ANOVA results showed that mean total Re was not significantly different (p > 0.05) between stations in 2020 but was significantly different (p <0.05) for West vs. South and North vs. South wells in 2021. Chapter two quantified quarterly (Q1 to Q4) soil water balance components evaluated annual Re and investigated multi-annual soil moisture dynamics in IRR_FLD and N_IRR_FLD pasture fields to determine quarterly and yearly variations and impacts from irrigation and precipitation. The two-year study took place from 01 April 2020 to 31 March 2022, and each year was divided into quarters to observe quarterly variations in the SWBM. Multi-year soil moisture variability was analyzed using the IRR_FLD and N_IRR_FLD South stations soil moisture data as they were the longest running stations for each field. Data from four soil moisture stations and a monitoring well were used at each field to calculate various field water budget components. The SCS-Curve Number method with a slope correction was used to estimate runoff in IRR_FLD and N_IRR_FLD. Results showed that winter P was the most significant water input for both fields and caused the most amount of DP during the year. For both fields, AET was the largest water output per year, ranging from 383 mm to 479 mm. RO was higher in IRR_FLD than in N_IRR_FLD and was likely caused by the summer irrigation that kept the soils near saturation in IRR_FLD and resulted in an earlier start to RO once the winter season began. Despite N_IRR_FLD having no DP during the summer, DP and S Kruskal Wallis comparisons between the two fields and the IRR and rainy season showed that there were no significant differences in S (p > 0.05), and similar results were found with DP (p > 0.05). The two exceptions were between the N_IRR_FLD winter season DP vs. N_IRR_FLD IRR season DP and the IRR_FLD winter season DP vs. N_IRR_FLD IRR season DP where there were significant differences (p < 0.05). Annual Re estimates varied more in IRR_FLD (289 mm in 2021 vs. 428 mm in 2022) than in N_IRR_FLD (157 mm vs. 178 mm). IRR season Re was higher than winter Re in IRR_FLD but not for N_IRR_FLD for 2021. TWA was also higher in IRR_FLD than in N_IRR_FLD. Groundwater levels differed between the two fields, with large peaks occurring during the irrigation season in IRR_FLD. Once the winter season began IRR_FLD groundwater level responded immediately, and the groundwater level for all but the South station remained elevated throughout the winter. In contrast, the groundwater level in N_IRR_FLD was slower to respond as it appeared the soil profile needed to be saturated before the groundwater level responded. The groundwater level for N_IRR_FLD also remained elevated throughout the winter. In the IRR_FLD South station soil water movement through the vadose zone was observed during both the IRR season and the rainy season. During the irrigation season, IRR events led to rises in soil moisture in at least one sensor, with the shallowest (0.2 m) sensor responding first, followed by the other two sensors. However, sensor response was not always sequential at the lower depths and changed depending on the IRR event and as the season progressed. Rapid rises on soil moisture were followed by drops as the soil drained and reached field capacity. Sensors’ response was sequential during the rainy season as soil water percolated down the soil profile. Apart from longer periods without rain, soils remained saturated throughout the winter and soil moisture remained high. In contrast, N_IRR_FLD South station soil moisture remained low throughout the summer as no irrigation inputs took placed. During the winter, soil moisture trends were similar to those observed in IRR_FLD. This project contributed to the understanding of the relationships between precipitation, irrigation, soil moisture, and shallow aquifer recharge in clayey pasture fields with fine-textured soils in the Pacific Northwest region of the United States. Based on soil physical properties, soil water, and groundwater level response onsite, and the results from other research near our study site, it is likely that macropore pathways played an important role in shallow aquifer recharge. However, their impact on DP and Re will need to be further explored. Summer irrigation in the clayey pasture field maintained an elevated groundwater level, potentially providing irrigation return flows to the riparian zone of a nearby stream. This suggests that summer irrigation could contribute return flow during the critical summer months. This research also showed that precipitation and irrigation patterns greatly impact soil moisture annual cycles, which lead to soil water percolation and groundwater recharge. These findings can aid water managers and farmers in similar fine-textured pasturelands in determining the impact that water inputs have on solute transport, soil water storage, groundwater recharge, and possibly streamflow augmentation.
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