Northern High Plains Research Articles

Spatial Variation in Transit Time Distributions of Groundwater Discharge to a Stream Overlying the Northern High Plains Aquifer, Nebraska, USA

AbstractGroundwater transit time distributions (TTDs) describe the spectrum of flow‐weighted apparent ages of groundwater from aquifer recharge to discharge. Regional‐scale TTDs in stream baseflow are often estimated from numerical models with limited calibration from groundwater sampling and suggest much younger groundwater discharge than has been observed by discrete age‐dating techniques. We investigate both local and regional‐scale groundwater TTDs in the Upper Middle Loup watershed (5,440 km2) overlying the High Plains Aquifer in the Nebraska Sand Hills, USA. We determined flow‐weighted apparent ages of groundwater discharging through the streambed at 88 discrete points along a 99 km groundwater‐dominated stream segment using 3H, noble gases, 14C, and groundwater flux measurements at the point‐scale (<7.6 cm diameter). Points were organized in transects across the stream width (3–10 points per transect) and transects were clustered in five sampling areas (10–610 m in stream length) located at increasing distances along the stream. Groundwater apparent ages ranged from 0 to 8,200 years and the mean groundwater transit time along the 99 km stream is >3,000 years. TTDs from upstream sampling areas were best fit by distributions with a narrow range of apparent ages, but when older groundwater from downstream sampling areas is included, the regional TTD is scale dependent and the distribution is better described by a gamma model (α ≈ 0.4) which accommodates large fractions of millennial‐aged groundwater. Observations indicate: (a) TTDs can exhibit spatial variability within a watershed and (b) watersheds can discharge larger fractions of old groundwater (>1,000 years) than commonly assumed.

Simulation of groundwater-flow dynamics in the U.S. Northern High Plains driven by multi-model estimates of surficial aquifer recharge

There is growing interest in incorporating higher-resolution groundwater modeling within the framework of large-scale land surface models (LSMs), including processes such as three-dimensional flow, variable soil saturation, and surface water/groundwater interactions. Conversely, complex groundwater models (e.g., the U.S. Geological Survey Groundwater-Flow Model, MODFLOW) often use simpler representations of land surface dynamics (e.g., surface vegetation, evapotranspiration, recharge) and may benefit from higher process fidelity and temporal resolutions in these inputs. This study investigates the potential of improving groundwater representation in LSMs and land surface dynamics in MODFLOW through forcing MODFLOW with recharge from LSMs. Groundwater simulations build on an existing and well-calibrated MODFLOW model of the U.S. Northern High Plains aquifer, a hydrologically complex basin under the dual impacts of conversion of native vegetation to intense irrigated agricultural fields and climate change. Simulated groundwater recharge from four different land models are used to drive MODFLOW groundwater simulations. Results show relatively large discrepancies between recharge estimates among simulations. Forcing MODFLOW using recharge simulated by some of the LSMs in place of a simple water balance model marginally improves MODFLOW groundwater simulation. Further, our results support the efficacy of coupling LSMs to a sophisticated groundwater model such as MODFLOW. The coupling results in notable improvements in matching the historical groundwater levels through reduction of the skewness coefficient in percent bias histogram (from 1.50 and 1.41 in original LSMs to 0.44 and 0.27, respectively, when MODFLOW is forced by groundwater recharge from LSMs) and reduction of bias. This modeling effort seeks to identify the best compromise between comprehensive land surface processes from global LSMs and advanced representation of groundwater from regional models.

Improving semi-arid agroecosystem services with cover crop mixes.

Winter wheat (Triticum aestivum, L.) production in the semi-arid US Northern High Plains (NHP) is challenged by frequent droughts and water-limited, low fertility soils. Composted cattle manure (compost) and cover crops (CC) are known to provide agroecosystem services such as improved soil health, and in the CC case, increased plant diversity, and competition with weedy species. The main concern of planting CC in winter wheat fallow rotation in regions that are more productive than the NHP, however, is the soil moisture depletion. It is unknown however, whether addition of CC to compost-amended soils in the NHP will improve soil properties and agroecosystem health without compromising already low soil water content. The main objective of this study was to assess the effects of four CC treatments amended with compost (45 Mg ha-1) or inorganic fertilizer (IF) (.09 Mg ha-1 mono-ammonium phosphate, 11-52-0 and 1.2 Mg ha-1ammonium sulfate, 21-0-0) on the presence of weeds, soil and plant total carbon (C), nitrogen (N), and biological dinitrogen (N2) fixation (BNF). Mycorrhizal Mix (MM), Nitrogen Fixer Mix (NF), Soil Building Mix (SB), a monoculture of phacelia (Phacelia tanacetifolia Benth L.) (PH), and a no CC control (no CC) were grown in native soil kept at 7% soil moisture in a greenhouse for a period of nine weeks. When amended with compost, MM was the most beneficial (48 g m-2 BNF and 1.7% soil C increase). SB had the highest germination, aboveground biomass, and decreased weed biomass by 60%. It also demonstrated the second highest amount of BNF (40 g m-2) and soil C increase by 1.5%. On contrary, IF hindered BNF by almost 70% in all legume-containing CC treatments and reduced soil C by 15%.

Influence of seasonal climatic water deficit and crop prices on rainfed crop grain harvest, repurposing, and abandonment in the western U.S.A.

Increasing climate aridity and drought, exacerbated by global warming, are increasing risks for western United States of America (U.S.A.) rainfed farming, and challenging producers’ capacity to maintain production and profitability. With agricultural water demand in the region exceeding limited supplies and fewer opportunities to develop new water sources, rainfed agriculture is under increasing pressure to meet the nation’s growing food demands. This study examines three major western U.S.A. rainfed crops: barley, spring wheat, and winter wheat. We analyzed the relationship between crop repurposing (the ratio of acres harvested for grain to the total planted acres) to seasonal climatic water deficit (CWD). To isolate the climate signal from economic factors, our analysis accounted for the influence of crop prices on grain harvest. We used historical climate and agricultural data between 1958 and 2020 to model crop repurposing (e.g. forage) across the observed CWD record using a fixed effect model. Our methodology is applicable for any region and incorporates regional differences in farming and economic drivers. Our results indicate that farmers are less likely to harvest barley and spring wheat for grain when the spring CWD is above average. Of the major winter wheat growing regions, only the Northern High Plains in Texas showed a trend of decreasing grain harvest during high CWD. For the majority of major crop growing regions, grain prices increased with lower levels of grain harvest. Interestingly, winter wheat repurposing is significantly higher in the southern Great Plains (∼50% harvested for grain) compared to the rest of the West (∼90%). Our results highlight that the major barley and spring wheat regions’ grain harvests are vulnerable to high spring CWD and low summer CWD, while winter wheat grain harvest is unaffected by variable CWD in most of the West.

Sunn hemp biomass accumulation, regrowth, and nutritive value in response to harvest time and cutting height

AbstractThe integrated crop‐livestock systems of the northern High Plains are lacking in annual legumes that meet the nutrient demands of beef cattle when alfalfa (Medicago sativa) is in limited supply. We investigated the biomass accumulation, regrowth biomass, and nutritive value of sunn hemp (Crotalaria juncea) in response to initial harvest day and cutting height in irrigated and dryland studies in Wyoming. Net biomass accumulations in 105‐day growing period were 4.4 tons acre−1 under irrigation and 0.7 tons acre−1 in dryland conditions. Initial harvest day after planting (iDAP) affected initial and regrowth biomass accumulation but did not affect net biomass accumulation in irrigated or dryland studies. Regrowth and net biomass accumulations were affected by cutting height in the irrigated study only. Nutritive value concentrations were significantly affected by iDAP in both irrigated and dryland studies. Under irrigation, net nutrient accumulation was not affected by iDAP but was significantly greater with a cutting height of 4.2 inches compared to 2.5‐ and 6‐inch heights. In contrast, the dryland study, net nutrient accumulation was not affected by cutting height but was higher in 55–105 iDAP than 45 iDAP. In the irrigated study, a cutting height of 4.2 inches produced net accumulations higher in both biomass accumulation and nutritive value. In the dryland study a harvest time of 55–105 iDAP produced the highest net accumulation regardless of harvest time or cutting height. Sunn hemp can be harvested once or twice in a 105‐day growing season to produce biomass and essential nutrients for livestock feeding.

Impacts of Ongoing Land-Use Change on Watershed Hydrology and Crop Production Using an Improved SWAT Model

The southern Ogallala Aquifer continues to deplete due to decades of irrigation with minimal recharge. Recently enacted regulations limiting groundwater withdrawals and the potential for farm profitability with cotton production systems indicate driving forces for increased cotton production acreage in the Northern High Plains of Texas (NHPT). This study focused on evaluating the land-use change from corn or winter wheat to cotton under irrigation and dryland conditions in the Palo Duro watershed (PDW) in the NHPT using an improved Soil and Water Assessment Tool (SWAT) model. Land-use change from irrigated corn to irrigated cotton led to reductions in average (2000–2014) annual irrigation, actual evapotranspiration (ETa), and surface runoff by 21%, 7%, and 63%, respectively. Nevertheless, the replacement of irrigated wheat with irrigated cotton caused irrigation and ETa to increase by 46% and 18%, respectively. Land-use conversion from dryland wheat to dryland cotton showed 0.1% and 15% decreases in ETa and surface runoff, respectively. More than 40% reductions in simulated cotton yields were found when the cotton planting area was moving northward to the cooler NHPT. The ongoing change in land use provided an option to lengthen the water availability of the southern Ogallala Aquifer for irrigation.

Reconstructing late Miocene depositional environments in the central High Plains, USA: Lithofacies and architectural elements of the Ogallala Formation

The late Miocene Ogallala Formation hosts much of the High Plains aquifer of North America, though it is poorly studied in the central High Plains region where only a small fraction of its total thickness is exposed. One exception is in western Kansas, where up to 40 m of the Ogallala Formation crop out along the bluffs of Ladder Creek Canyon. These deposits consist of stacked and laterally extensive, multistory channel bodies dominated by sandy bedforms with little or no intervening floodplain mudstone. We interpret each ~2–4 m-thick story as a sandstone bed that formed during a single depositional episode and infilled a broad and relatively shallow braided river channel. Each story preserves elements suggesting high discharge conditions, followed by low discharge or the complete abandonment of the river channel after active streamflow either migrated away from the study area or ceased regionally for a relatively long period of time. High discharge conditions are characterized by channel-filling, current-formed transverse bars, gravel-rich longitudinal bars, and sand sheets deposited while the channel was largely flooded and nearly the entire riverbed mobilized. Low discharge conditions are indicated by lens-shaped, trough cross-stratified channels scoured into previously deposited bars. Channel-fill deposits were formed when the remaining flow was confined to one or more narrow streams that braided across the exposed channel belt. Persistent low discharge conditions are suggested by channel-belt-wide subaerial exposure of bar surfaces for a period of time sufficient to promote colonization by soil-burrowing organisms and moderate soil development. Abandonment of the broader channel belt is indicated by advanced calcretization within a story and the presence of aerially restricted and fossiliferous mud-filled channel pools where the final repositories of surface water attracted local paleofauna before infilling or drying completely. The depositional environment of the Ogallala Formation in the study area is most similar to “Platte type” fluvial systems characterized as shallow, perennial, and sand-dominated braided rivers. Comparisons with previously studied localities in the northern and southern High Plains reveal differences in fluvial style, sediment source areas, cut-and-fill geometries, and eolian input.

Agricultural Irrigation Effects on Hydrological Processes in the United States Northern High Plains Aquifer Simulated by the Coupled SWAT-MODFLOW System

Groundwater use for irrigation has a major influence on agricultural productivity and local water resources. This study evaluated the groundwater irrigation schemes, SWAT auto-irrigation scheduling based on plant water stress (Auto-Irr), and prescribed irrigation based on well pumping rates in MODFLOW (Well-Irr), in the U.S. Northern High Plains (NHP) aquifer using coupled SWAT-MODFLOW model simulations for the period 1982–2008. Auto-Irr generally performed better than Well-Irr in simulating groundwater irrigation volume (reducing the mean bias from 86 to −30%) and groundwater level (reducing the normalized root-mean-square-error from 13.55 to 12.47%) across the NHP, as well as streamflow interannual variations at two stations (increasing NSE from 0.51, 0.51 to 0.55, 0.53). We also examined the effects of groundwater irrigation on the water cycle. Based on simulation results from Auto-Irr, historical irrigation led to significant recharge along the Elkhorn and Platte rivers. On average over the entire NHP, irrigation increased surface runoff, evapotranspiration, soil moisture and groundwater recharge by 21.3%, 4.0%, 2.5% and 1.5%, respectively. Irrigation improved crop water productivity by nearly 27.2% for corn and 23.8% for soybean. Therefore, designing sustainable irrigation practices to enhance crop productivity must consider both regional landscape characteristics and downstream hydrological consequences.

Sandstone-body geometry and hydrostratigraphy of the northern High Plains Aquifer system, USA

Cenozoic sandstones of the North American interior host the voluminous High Plains Aquifer (HPA) system, which supplies water to c. 30% of all irrigated farmland in the USA. Sandstone bodies in the HPA have eluded characterization because the correlation of discrete sedimentary units between boreholes is exceedingly difficult or even impossible. Our work, however, demonstrates that sandstone-body geometry can be characterized at regional, if not basinal scales, using multi-resolution methods, including remote sensing, outcrop mapping, borehole geophysics, and airborne electromagnetics. These and other scientific advances suggest that the HPA contains a complex assemblage of fluvial sandstone bodies with predictable dimensions. Hydrologic property contrasts between these sandstones and the enclosing, finer-grained strata define a complex arrangement of hydrostratigraphic units. We present this evolving understanding of HPA hydrostratigraphy in a new conceptual model that aids in the understanding of water-supply challenges. We maintain that sustaining well yields and aquifer productivity into the future depends on mapping the architecture of sandstone bodies in the subsurface using advanced techniques. Thematic collection: This article is part of the Hydrogeology of Sandstone collection available at: https://www.lyellcollection.org/cc/hydrogeology-of-sandstone

Simulating the climate change impacts and evaluating potential adaptation strategies for irrigated corn production in Northern High Plains of Texas

Sustaining irrigated corn (Zea mays L.) production under changing climate and reduced irrigation water availability presents a key challenge for producers in the Northern High Plains (NHP) of Texas. We assessed climate change impacts on corn production at Bushland in the NHP region using the CERES-Maize model under 36 future climate scenarios. These scenarios included nine global climate models (GCMs), two representative concentration pathways (RCPs) 4.5 and 8.5, and two future time periods 2050s (2036–2065) and 2080s (2066–2095). Simulated grain corn yield decreased, under all scenarios in the future, by approximately 31 % on average in the 2050s and by approximately 55 % in the 2080s, under RCP 8.5, mainly due to reduced unit grain weight and biomass, and shorter crop cycle. Seasonal irrigation water use was simulated to either increase or decrease in the future depending on the rainfall projection of a GCM. Adaptation strategies considered in this study included using ideotypes with a longer maturity, higher yield potential, and greater heat tolerance than the reference cultivar, and shifting of planting dates. The ideotypes were created by modifying thermal times from emergence to end-of-juvenile phase, thermal time from silking to maturity, maximum kernel number per plant, radiation use efficiency (RUE), and optimum and failure temperature thresholds during relative grain filling. The planting dates considered were 16 April, 1 May, 16 May (reference), 1 June, and 16 June. By increasing the reproductive period by 13 days, grain yield could increase by 40 %, but that could increase seasonal irrigation water requirement by 10 %. Grain corn yield increased by 13 % on average when the maximum number of kernels per plant or RUE were increased by 15 %, with minor changes in seasonal irrigation water requirement. Heat tolerant ideotypes showed yield advantage over the reference cultivar without much change in irrigation. Delayed planting increased grain yield in the future. Overall, grain yield could decline under climate change without adaptation. Ideotypes with greater silking to maturity period, potential number of kernels per plant, RUE, and heat tolerance during grain filling, showed yield benefits in the future.

Sunn hemp planting date effect on growth, biomass accumulation, and nutritive value in southeastern Wyoming

AbstractSustainablesuccess of the Northern High Plains (NHP) crop and livestock industry is limited by market accessibility, infertile soils, short growing seasons, and availability of high nutritive value, low‐cost feed. Adoption of the tropical legume, Crotalaria juncea L. (commonly known as sunn hemp), into NHP cropping systems could improve the sustainability of the rotation by fitting into summer growing windows and being used as a hay alternative. Rainfed and irrigated studies were conducted in Lingle, WY, to determine the target planting date for sunn hemp in the NHP. Later planting day of year was correlated with increased percent establishment in the rainfed study but had no effect in the irrigated study. In both irrigated and rainfed studies, planting between 8 June and 18 June maximized biomass accumulation. Biomass accumulation 60 d after planting ranged from 0.2 to 5.1 Mg ha–1 in the rainfed study and 1.2 to 4.2 Mg ha–1 in the irrigated study. For both irrigated and rainfed studies, the nutritive value of sunn hemp was high with crude protein greater than 220 g Kg–1, acid detergent fiber less than 305 g Kg–1, neutral detergent fiber less than 331 g Kg–1, relative feed values above 183, and total digestible nutrients greater than 590 g Kg–1. Planting date did not affect nutritive value. Sunn hemp grown in the NHP has a 60‐d yield potential 5.1 Mg ha–1, should be planted between 8–18 June to 60‐d biomass production, and has nutritive value equivalent to supreme quality alfalfa (Medicago sativa L.) hay.

Featured Collection Introduction: National Water Model III

Featured Collection Introduction: National Water Model III

Common Pool Resource Management: Assessing Water Resources Planning for Hydrologically Connected Surface and Groundwater Systems

Common pool resource (CPR) management has the potential to overcome the collective action dilemma, defined as the tendency for individual users to exploit natural resources and contribute to a tragedy of the commons. Design principles associated with effective CPR management help to ensure that arrangements work to the mutual benefit of water users. This study contributes to current research on CPR management by examining the process of implementing integrated management planning through the lens of CPR design principles. Integrated management plans facilitate the management of a complex common pool resource, ground and surface water resources having a hydrological connection. Water governance structures were evaluated through the use of participatory methods and observed records of interannual changes in rainfall, evapotranspiration, and ground water levels across the Northern High Plains. The findings, documented in statutes, field interviews and observed hydrologic variables, point to the potential for addressing large-scale collective action dilemmas, while building on the strengths of local control and participation. The feasibility of a “bottom up” system to foster groundwater resilience was evidenced by reductions in groundwater depths of 2 m in less than a decade.

Evaluating Baseflow Simulation in the National Water Model: A Case Study in the Northern High Plains Region, USA

AbstractThe National Water Model (NWM) is a high‐resolution hydrological model capable of providing streamflow forecast at 2.7 million reaches across the conterminous United States (U.S.). It utilizes a conceptual (not physically explicit) module for estimating groundwater discharge (baseflow) to streams, and the baseflow module only allows for one‐way flux from the aquifer to the streams, with no interflow between surficial or groundwater catchments. This study evaluated the ability of the NWM to simulate baseflow in different geologic conditions with a case study in the Northern High Plains region. The study also evaluated two alternative formulations for the baseflow module in the NWM with the goal of improving simulated baseflow accuracy. The NWM simulated baseflow more accurately in a clayey catchment than in a sandy catchment. The model, however, over simulated baseflow during storm runoff events and under simulated during low flow events in both catchments. Analysis of groundwater recharge simulation showed that the NWM over simulated groundwater recharge for both catchment types, and reducing groundwater recharge using U.S. Geological Survey Baseflow Indexes improved the model performance for baseflow simulation, especially in the sandy catchment. Evaluation of the alteranative formulations showed that a modified Rorabaugh‐Rutledge formulation was able to improve the baseflow simulation with RMSE of 7.98 m3/s and R2 of 0.41 and also improved the timing of peak flows.

Effects of Groundwater‐Surface Water Exchange Mechanism in the National Water Model over the Northern High Plains Aquifer, USA

AbstractSurface water and groundwater are intimately connected by a two‐way flux between the stream and underlying aquifers. The National Water Model (NWM) currently only considers a one‐way flux, where groundwater can enter a stream but cannot return to the aquifer. The Northern High Plains Aquifer, USA is used as a case study to investigate the consequences of omitting two‐way stream–aquifer fluxes on streamflow prediction capabilities of the NWM during hydrologic extremes. Instead of traditional field techniques to identify stream–aquifer fluxes, this study presents an integrated approach to classify likely stream regimes using three identification methods: United States Geological Survey (USGS) gage data, simulated stream–aquifer fluxes from an existing USGS Groundwater Availability Model, and the normalized difference vegetation index from remote sensing. For flood events, the modeled flood response for losing streams is characterized by statistically significant earlier peak discharges and an overestimate of the observed flood volume when compared to gaining streams. For drought events, our study found no statistical difference between losing and gaining streams, however, modeled streamflow from the NWM overestimated the observed USGS hydrograph. The systematic overestimate of streamflow by the NWM could be, in part, due to the lack of a losing stream mechanism which was on average 0.1% streamflow loss per streamwise km along the river.

Sensitivity analysis of data-driven groundwater forecasts to hydroclimatic controls in irrigated croplands

In the last decades, advancements in computational science have greatly expanded the use of artificial neural networks (ANNs) in hydrogeology, including applications on groundwater forecast, variable selection, extended lead-times, and regime-specific analysis. However, ANN-model performance often omits the sensitivity to observational uncertainties in hydroclimate forcings. The goal of this paper is to implement a data-driven modeling framework for assessing the sensitivity of ANN-based groundwater forecasts to the uncertainties in observational inputs across space, time, and hydrological regimes. The objectives are two-folded. The first objective is to couple an ANN model with the PAWN sensitivity analysis (SA). The second objective is to evaluate the scale- and process-dependent sensitivities of groundwater forecasts to hydroclimate inputs, computing the sensitivity index in groundwater wells (1) across the whole time-series (for the global sensitivity analysis); (2) across the output sub-regions with conditions of water deficit and water surplus (for the ‘regional’ sensitivity analysis); and (3) at each time step (for the time-varying sensitivity analysis). The implementation of the ANN-PAWN occurs in 68 wells across the Northern High Plains aquifer, USA, with pre-time-step rainfall, evapotranspiration, snowmelt, streamflow, and groundwater measurements as inputs. Results show that evapotranspiration and rainfall are the major sources of uncertainty, with the latter being particularly relevant in water surplus conditions and the former in water deficit conditions. The time-varying sensitivity analysis leads to the identification of localized sensitivities to other sources of uncertainty, as snowmelt in spring or river flow during the annual peak period at the groundwater level.

Adapting a Cloud-Based Irrigation Scheduler for Sugar Beets in the High Plains

Highlights An existing sugar beet crop coefficient curve (K cr ) was modified to better represent canopy development in northeast Colorado conditions. The modified K cr curve improved the estimated soil water deficits (net irrigation requirements) calculated by the cloud-based Water Irrigation Scheduler for Efficient Application (WISE App). Feedback from sugar beet growers and agronomists helped expand WISE applicability in the northern High Plains with access to additional weather station networks and functionality to aggregate irrigation data across multiple sugar beet fields or regions. Abstract . The convergence of agrometeorological network, database, and cloud-computing technologies has enabled greater accessibility of irrigation management tools for growers. The goal of this research and outreach project was to adapt an existing cloud-based irrigation scheduler (WISE) for use by sugar beet (Beta vulgaris L.) producers in eastern Colorado and a wider area of a cooperative operating in Colorado, Nebraska, Wyoming, and Montana. Four center pivot sugar beet fields in northeast Colorado were monitored during the 2013 and 2014 growing seasons. Soil water, leaf area index (LAI), and weather data were used to estimate the soil water deficit (net irrigation requirement) and to modify a crop coefficient (Kcr) curve originally reported in the literature based on growing degree days (GDD). The average cumulative GDDs for sugar beets to mature (100% maturity) was 2,944°C·d. The localized Kcr had a peak value (Kcr,mid) occurring between 43% and 69% of maturity, which corresponded to effective full cover (LAI = 3) and start of leaf senescence, respectively. In contrast, the original Kcr curve from literature had a longer duration of Kcr,mid spanning 33% to 83% of maturity. Use of the modified Kcr curve in lieu of the original Kcr curve in WISE reduced the relative error of soil water deficits by 12% to 35%. Feedback and collaborations from representative sugar beet growers and agronomists in the Western Sugar Cooperative led to expansion of WISE weather data access in the High Plains to include sugar beet growing areas in western Nebraska, eastern and northern Wyoming, and southern Montana. Keywords: Crop coefficient, Evapotranspiration, Irrigation scheduling, Soil water balance, Soil water deficit, Sugar beets.

Estimating High-Resolution Groundwater Storage from GRACE: A Random Forest Approach

Gravity Recovery and Climate Experiment (GRACE) data have become a widely used global dataset for evaluating the variability in groundwater storage for the different major aquifers. Moreover, the application of GRACE has been constrained to the local scale due to lower spatial resolution. The current study proposes Random Forest (RF), a recently developed unsupervised machine learning method, to downscale a GRACE-derived groundwater storage anomaly (GWSA) from 1° × 1° to 0.25° × 0.25° in the Northern High Plains aquifer. The RF algorithm integrated GRACE to other satellite-based geospatial and hydro-climatological variables, obtained from the Noah land surface model, to generate a high-resolution GWSA map for the period 2009 to 2016. This RF approach replicates local groundwater variability (the combined effect of climatic and human impacts) with acceptable Pearson correlation (0.58 ~ 0.84), percentage bias (−14.67 ~ 2.85), root mean square error (15.53 ~ 46.69 mm), and Nash-Sutcliffe efficiency (0.58 ~ 0.84). This developed RF model has significant potential to generate finer scale GWSA maps for managing groundwater at both local and regional scales, especially for areas with sparse groundwater monitoring wells.

Simulating the impacts of climate change on hydrology and crop production in the Northern High Plains of Texas using an improved SWAT model

Modeling the effects of climate change on hydrology and crop yield provides opportunities for choosing appropriate crops for adapting to climate change. In this study, climate change impacts on irrigated corn and sorghum, dryland (rainfed) sorghum, and continuous fallow in the Northern High Plains of Texas were evaluated using an improved Soil and Water Assessment Tool (SWAT) model equipped with management allowed depletion (MAD) irrigation scheduling. Projected climate data (2020–2099) from the Coupled Model Intercomparison Project Phase 5 (CMIP 5) of 19 General Circulation Models (GCMs) were used. Climate data were divided into four 20-year periods of near future (2020–2039), middle (2040–2059), late (2060–2079), and end (2080–2099) of the 21st century under two Representative Concentration Pathway (RCP) emission scenarios (RCP 4.5 and RCP 8.5). For irrigated corn, median annual crop evapotranspiration (ET) and irrigation decreased by 8%–25% and 15%–42%, respectively, under the climate change scenarios compared to the historical period (2001–2010). The median yield was reduced by 3%–22% with exponentially decreases in the latter half of the 21st century. For sorghum, the reduction of median annual crop ET ranged from 6%–27%. However, the decline in the median annual irrigation was within 15%, except for the 2060–2079 and 2080–2099 periods under RCP 8.5 scenarios with 30% and 49% reductions in median annual irrigation. The median irrigated sorghum yield declined by 6%–42%. The median annual crop ET of dryland sorghum decreased by 10%–16%. The reduction in median yield was within 10% of the historical dryland sorghum yield. The decrease in median annual evaporation varied from 15%–23% under future continuous fallow conditions. The elevated CO2 level of future climate scenarios was the primary factor for the decrease in the ET and irrigation. The reduction in future crop yield was mainly attributed to the shortening of the maturity period caused by increased future temperature.

Attribution of the 2017 Northern High Plains Drought

This study investigates the physical processes that led to the 2017 northern High Plains drought, particularly the roles of SST anomalies and atmospheric internal variability. It also assesses the impact of global warming on the frequency of drought occurrence.