Evaluating factors that control recharge in a thick vadose zone under climate variability and change

Abstract

Groundwater is an important freshwater resource, especially in areas that rely almost completely on groundwater and where overdraft conditions can occur. Climate variability and change pose uncertainties in the sustainability of groundwater resources. Understanding how factors including land use/land cover (LULC), climate conditions (precipitation, air temperature), and recharge mechanisms (diffuse and irrigation recharge) respond to climate variability and change can help improve the inflow component of future groundwater budgets and inform groundwater sustainability planning and related policy decisions. In this study, I used Hydrus-1D to model vadose zone flux for six sites in the Central Platte Natural Resources District (CPNRD) of central Nebraska. The six sites represent spatial and temporal climate variations and the dominant recharge mechanisms (diffuse and irrigation) under LULC of the study area. Historical recharge and total potential profiles were simulated in Hydrus-1D for the years 1950-2018 and future recharge and total potential profiles were simulated in Hydrus-1D for the years 1950-2100 using an ensemble of nine downscaled global climate models (GCMs) at representative concentration pathways (RCPs) 4.5 and 8.5. Future recharge showed more recharge under irrigated sites compared to rangeland sites in their respective locations (west and east). Future recharge was also projected to decrease over time, with larger decreases under RCP 8.5. Historical recharge was compared to historical periods of increased precipitation and drought and a Palmer Drought Severity Index (PDSI) timeseries adjusted for lag correlations. Findings showed recharge correlated with PDSI and LULC controlled the recharge response time; corn crop sites within 20-24 months where rangeland sites responded within 58-372 months. Total potential profiles were calculated using head at depth outputs from Hydrus-1D. Historic total potential profiles for each site representing dates surrounding the most recent wet and dry period showed spatial variations in precipitation to be a controlling factor of total potential responses to climate variability. Western sites that receive less average annual precipitation showed less downward flux during dry periods and more downward flux during wet periods, whereas eastern sites that receive more average annual precipitation showed less downward flux during wet periods compared to dry periods but more upward seasonal flux during dry periods. Future total potential profiles were calculated using one of the GCMs at both RCPs 4.5 and 8.5 for the years 2040 and 2099 where findings showed more seasonal upward flux at all sites in the year 2099 under RCP 8.5.