Thicketized oak woodlands reduce groundwater recharge

Woodlands and pastures across the Post Oak Savannas (POS) in Texas have been undergoing thicketization over the last century via encroachment by understory shrubs such as Yaupon (Ilex decidua, Ilex vomitoria) and expansion of eastern redcedar (Juniperus virginiana). Because a large part of POS overlies the Carrizo-Wilcox (CW) aquifer - the third most important aquifer in Texas, there is a strong incentive to identify opportunities to increase groundwater recharge through land management. The purpose of this research is to evaluate the influence of thicketization of post oak (Quercus stellata) stands on deep drainage (DD) in POS. We achieved this by, a) applying chloride mass balance on soil cores, and b) simultaneously monitoring soil moisture in a woodland pasture setting in POS. Four sites representing different vegetation covers were identified for sampling: 1) a thicketized oak woodland paired with an adjacent open site, 2) a woodland mosaic, 3) a pasture and 4) a pine-oak stand paired with an adjacent open site. A total of 24 soil cores to the depth of 260 cm were collected and (soil) pore water chloride concentrations at multiple depths were measured. Soil moisture was monitored at 21 locations, to the depth of 140-260 cm using a neutron moisture meter. Negligible DD was estimated in the thicketized woodland, whereas most open locations recorded 3-18 cm/year and the woodland mosaic 0-1 cm of DD. Soil moisture data, collected from Jul-2020 to Jun-2021 also suggested higher deep drainage fluxes under open areas - with occurrence of sub-surface saturation only under the open areas and never under the woodlands. These results suggest that the thicketization in oak savannas is substantially reducing groundwater recharge. Given the extent of thicketized oak savannas across United States, this could be impacting water budgets and groundwater recharge rates on regional scales.

Australian gridded chloride deposition-rate dataset

Chloride deposition-rate measurements at points within Australia are upscaled to the entire continent on a regular 0.05° grid. The upscaling uses a double-exponential correlation between deposition rate and distance to the coast, where the parameters in the double-exponential are spatially varying. These parameters are estimated using least-squares with Tikhonov regularisation to ensure minimal spatial variability. A calibration-constrained, null-space Monte-Carlo analysis is used to quantify uncertainty in the prediction. The resulting dataset consists of the best-fit chloride deposition rates across Australia, as well as estimates of uncertainty. The dataset can be used for various purposes including: estimating groundwater recharge through the use of the chloride mass-balance method; catchment salt balance estimates; regional investigations of groundwater hydrochemistry; and, corrosion prediction.

Chloride inputs to the North Saskatchewan River watershed: the role of road salts as a potential driver of salinization downstream of North America's northern most major city (Edmonton, Canada)

Multiple anthropogenic activities are driving the salinization of freshwater environments threatening water resources worldwide. Accordingly, this research will first examine the spatial and temporal variability of major ions (i.e. Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄^2-, CO₃^2-, and HCO₃^-) upstream and downstream of the northernmost major city in North America (Edmonton, Canada). Second, this research will estimate the relative contributions of the major sources of chloride (Cl), the main constituent of road deicers, to the sub-basin around Edmonton. Monthly water quality data was for three sites on the North Saskatchewan River (NSR): Rocky Mountain House (RMH - downstream of the Rocky Mountain headwaters), Devon Bridge (upstream of Edmonton) and Pakan Bridge (downstream of Edmonton). Change ratios investigate the downstream alterations of major ions at Pakan and Devon, relative to RMH. Seasonal Kendall tests examine temporal trends (1987-2017). A mass-balance approach then quantifies the major sources of Cl in the sub-basin of the NSR between Devon and Pakan. Progressing from the Rocky Mountain headwaters (at RMH) to downstream of Edmonton (at Pakan), Cl^- increased by >5.5 times, Na⁺ by 4.8 times and K by 2.7 times. No significant temporal trends for Cl^-, Na⁺ and K⁺ were evident at Devon (upstream of Edmonton), whereas all three significantly increased at Pakan (downstream of Edmonton). Deicers (54%), agriculture (20%), along with waste water treatment plant and industrial effluent (15%) were the largest Cl sources in the NSR Devon-Pakan sub-basin. In total, 77 Gg yr^-1 of Cl (or 6 t km² yr^-1) is added to the Devon-Pakan sub-basin, of which, 43 Gg yr^-1 is retained. Understanding and managing the major drivers of freshwater salinization will be of increasing importance in the 21^st century owing to the potential salinization of freshwater resources in the context of a changing climate.

Land use change impacts on the amount and quality of recharge water in the loess tablelands of China

Exploring how land use change (LUC) influences the amount and quality of recharge water is important for groundwater sustainability and land use planning. With loess of up to 200m in thickness and unsaturated zones up to 100m below the surface, the loess tablelands in China can store abundant groundwater resources. However, groundwater depletion and substantial LUC have been simultaneously observed. It is thus necessary to investigate the relationship between LUC and groundwater. We sampled 10-m soil profiles for three land use types (farmlands, apple orchards of 10 and 20years old). After measuring the chloride and nitrate concentration in soil pore water, the LUC effects on the amount and quality of recharge water quality were quantified based on the mass balance method. Results showed that soil moisture in aged (20-year-old) apple orchards was significantly reduced relative to that measured in farmlands and younger (10-year-old) orchards, where measured soil moistures were roughly equal. The accumulated nitrate‑nitrogen and the depth below which nitrate is stable was smallest in farmlands, intermediate in 10-year-old apple orchards, and largest under 20-year-old apple orchards. The diffuse recharge was 33.0±17.9mmyear^-1, accounting for 7.3±1.8% of mean annual precipitation under farmlands; however, the conversion from farmlands to 20-year-old apple orchards reduced recharge by 42%. The nitrate infiltrating to groundwater annually was 4.9±2.9kghm^-2 and 4.1±3.1kghm^-2 under farmlands and 20-year-old apple orchards, respectively. The impacts of LUC over the past decades have not yet reached groundwater because of low recharge rates; further, the primary factor influencing groundwater quality is recharge rate rather than pollutant concentration. As such, the LUC from farmland to apple orchard has little impact on short-term groundwater recharge and quality; long term impact, however, may be significant.

Deep rooted apple trees decrease groundwater recharge in the highland region of the Loess Plateau, China

Unlike recharge in shallow rooted ecosystems, estimating the groundwater recharge beneath deep rooted plants that absorb water from deep soil remains difficult. The purpose of this research is to develop an approach to estimate the groundwater recharge beneath deep-rooted vegetation by combining water mass balance and chloride mass balance (CMB) and to quantify how the conversion of shallow-rooted cropland to deep-rooted apple orchards changes groundwater recharge. The proposed groundwater recharge rate under deep-rooted vegetation in this study is the difference between the groundwater recharge rate in a cropland (obtained using CMB) and the mean annual soil water storage deficit beneath an adjacent deep-rooted vegetation. The results show that the conversion from cropland (shallow-rooted) to apple orchard (deep-rooted) decreased soil water storage by 776, 1106, and 1117mm, corresponding to 19, 20, and 26-year-old apple orchards, respectively. Groundwater recharge beneath cropland, on average, was 58mmyr^-1, which amounts to 10% of the average annual precipitation. Groundwater recharge beneath the apple orchards were variable, but all being <3% of the average annual precipitation. The conversion of cropland to apple orchards lead to a substantial decrease in groundwater recharge, potentially threatening the sustainability of the land use change.

Integrating soil water and tracer balances, numerical modelling and GIS tools to estimate regional groundwater recharge: Application to the Alcadozo Aquifer System (SE Spain)

Groundwater recharge is one of the key variables for aquifer management and also one of the most difficult to be evaluated with acceptable accuracy. This is especially relevant in semiarid areas, where the processes involved in recharge are widely variable. Uncertainty should be estimated to know how reliable recharge estimations are. Groundwater recharge has been calculated in the Alcadozo Aquifer System, under steady state conditions, at regional (aquifer) and sub-regional (spring catchment) scales applying different methods. The regional distribution of long-term average recharge values has been estimated with the chloride mass balance method using data from four rain stations and 40 groundwater samples covering almost the whole aquifer surface. A remarkable spatial variability has been found. Average annual recharge rates ranges from 20 to 243mmyear(-1) across the aquifer, with an estimated coefficient of variation between 0.16 and 0.38. The average recharge/precipitation ratio decreases from 34% in the NW to 6% in the SE, following the topographic slope. At spring-catchment scale, recharge has been estimated by modelling the soil water balance with the code Visual Balan 2.0. The results, calibrated with discharge data of the two main springs Liétor and Ayna, are 35.5 and 50mmyear(-1) respectively, with estimated coefficients of variation of 0.49 and 0.36. A sensitivity analysis showed that soil parameters influence the most the uncertainty of recharge estimations. Recharge values estimated with both methods and at two temporal and spatial scales are consistent, considering the regional variability obtained with the chloride method and the respective confidence intervals. Evaluating the uncertainties of each method eased to compare their relative results and to check their agreement, which provided confidence to the values obtained. Thus, the use of independent methods together with their uncertainties is strongly recommended to constrain the magnitude and to provide reliance to recharge estimations.