Fred D. Tillman

Changes in groundwater recharge under projected climate in the upper Colorado River basin

Understanding groundwater-budget components, particularly groundwater recharge, is important to sustainably manage both groundwater and surface water supplies in the Colorado River basin now and in the future. This study quantifies projected changes in upper Colorado River basin (UCRB) groundwater recharge from recent historical (1950–2015) through future (2016–2099) time periods, using a distributed-parameter groundwater recharge model with downscaled climate data from 97 Coupled Model Intercomparison Project Phase 5 climate projections. Simulated future groundwater recharge in the UCRB is generally expected to be greater than the historical average in most decades. Increases in groundwater recharge in the UCRB are a consequence of projected increases in precipitation, offsetting reductions in recharge that would result from projected increased temperatures.

Understanding the past to interpret the future: comparison of simulated groundwater recharge in the upper Colorado River basin (USA) using observed and general-circulation-model historical climate data

In evaluating potential impacts of climate change on water resources, water managers seek to understand how future conditions may differ from the recent past. Studies of climate impacts on groundwater recharge often compare simulated recharge from future and historical time periods on an average monthly or overall average annual basis, or compare average recharge from future decades to that from a single recent decade. Baseline historical recharge estimates, which are compared with future conditions, are often from simulations using observed historical climate data. Comparison of average monthly results, average annual results, or even averaging over selected historical decades, may mask the true variability in historical results and lead to misinterpretation of future conditions. Comparison of future recharge results simulated using general circulation model (GCM) climate data to recharge results simulated using actual historical climate data may also result in an incomplete understanding of the likelihood of future changes. In this study, groundwater recharge is estimated in the upper Colorado River basin, USA, using a distributed-parameter soil-water balance groundwater recharge model for the period 1951–2010. Recharge simulations are performed using precipitation, maximum temperature, and minimum temperature data from observed climate data and from 97 CMIP5 (Coupled Model Intercomparison Project, phase 5) projections. Results indicate that average monthly and average annual simulated recharge are similar using observed and GCM climate data. However, 10-year moving-average recharge results show substantial differences between observed and simulated climate data, particularly during period 1970–2000, with much greater variability seen for results using observed climate data.RésuméEn évaluant les impacts potentiels du changement climatique sur les ressources en eau, les gestionnaires de l’eau cherchent à comprendre comment les conditions futures peuvent être différentes de celles d’un passé récent. Les études des impacts du climat sur la recharge des eaux souterraines comparent souvent la recharge simulée de périodes futures et de temps historique sur une moyenne mensuelle ou sur une base moyenne annuelle, ou comparent la recharge moyenne pour des décennies à venir à celle d’une décennie récente. Les estimations de la recharge historique de référence, qui sont comparées avec les conditions futures, sont souvent de simulations utilisant des données climatiques historiques observées. La comparaison des résultats moyens mensuels, des résultats moyens annuels, ou également faisant la moyenne sur des décennies historiques sélectionnées, pourrait masquer la véritable variabilité des résultats historiques et conduire à une interprétation erronée des conditions futures. La comparaison des résultats simulés de la recharge future en utilisant les données climatiques d’un modèle de circulation globale (GCM) aux résultats simulés de la recharge en utilisant les données climatiques historiques actuelles pourraient également conduire à une compréhension incomplète de la probabilité des changements futurs. Dans cette étude, la recharge des eaux souterraines est estimée dans le bassin supérieur de la rivière Colorado, aux Etats-Unis d’Amérique, en utilisant un modèle distribué des paramètres du sol et du bilan hydrique pour calculer la recharge des eaux souterraines pour la période 1951–2010. Les simulations de recharge sont réalisées en utilisant les données de précipitation, de température maximum, et de température minimum, à partir des données climatiques observées et des projections climatiques 97 CMIP5 (Projet d’Intercomparaison de Modèle Couplé, phase 5). Les résultats montrent que les recharges moyennes mensuelles et annuelles simulées sont similaires en utilisant les données climatiques observées ou simulées GCM. Toutefois, les résultats de la recharge moyenne glissante sur 10 ans montrent des différences significatives entre les données climatiques observées et simulées, en particulier durant la période 1970–2000, avec une variabilité beaucoup plus importante constatée pour les résultats utilisant les données climatiques observées.ResumenEn la evaluación de los posibles impactos del cambio climático sobre los recursos hídricos, los gestores del agua tratan de comprender cómo las futuras condiciones pueden diferir de las del pasado reciente. Los estudios de los impactos del clima sobre la recarga de agua subterránea a menudo comparan recargas simuladas a partir de períodos de tiempo futuros e históricos sobre una base promedios mensuales o anuales globales, o comparan la recarga media de las próximas décadas a que a partir de una sola década reciente. Las estimaciones de recarga histórica a partir de una línea de base, que se comparan con las condiciones futuras, son a menudo a partir de simulaciones utilizando datos climáticos históricos observados. La comparación de los resultados de promedio mensuales, los resultados de promedios anuales, o incluso un promedio histórico de más décadas seleccionadas, pueden enmascarar la verdadera variabilidad en los resultados históricos y dar lugar a una interpretación errónea de las condiciones futuras. La comparación de los futuros resultados de recarga simulados utilizando los datos climáticos del modelo de circulación general (GCM) con los resultados simulados utilizando datos climáticos históricos reales también puede dar lugar a una comprensión incompleta de la probabilidad de los cambios futuros. En este estudio se estima la recarga del agua subterránea en la cuenca alta del río Colorado, EEUU, utilizando un modelo de balance de agua del suelo de parámetro distribuido de recarga del agua subterránea para el período 1951–2010. Las simulaciones de la recarga se llevan a cabo utilizando los datos de precipitación, temperatura máxima y mínima a partir de datos climáticos observados y de proyecciones del 97 CMIP5 (Coupled Model Intercomparison Project, phase 5). Los resultados indican que la recarga promedio mensual y anual simulada son similares utilizando datos climáticos observados y el GCM. Sin embargo, utilizando los promedios móviles de 10 años los resultados de la recarga muestran diferencias sustanciales entre los datos climáticos observados y simulados, sobre todo durante período 1970–2000, apreciándose una mucho mayor variabilidad para los resultados a partir de los datos climáticos observados.摘要在评估气候变化对水资源的潜在影响中,水管理者寻求了解未来状况与近期有何不同。研究气候对地下水补给的影响常常在平均每月基础上或者每年基础上比较未来和历史时期的模拟补给量,或者比较未来几十年到最近10年的平均补给量。基线历史补给估算值与未来状况进行了比较,这个估算值常常通过采用观测的历史气候资料进行模拟而获取。平均月度结果、平均年度结果、或者甚至过去所选择的历史上数十年的平均数比较可能会掩盖历史结果的真实变化性,并且导致错误解译未来的状况。采用综合循环模型(GCM)气候资料模拟的未来补给结果与采用实际历史气候资料模拟的补给结果比较也可能会导致不能完全了解未来变化的可能性。在本项研究中,采用分布参数土壤-水平衡地下水补给模型对1951年到2010年间美国上科罗拉多河流域的地下水补给量进行了估算。利用观测的气候资料中以及97 CMIP5(耦合模型相互比较项目,第五阶段)预测结果中的降水、最高温度和最低温度资料进行了补给模拟实验。结果显示,采用观测的和综合循环模型气候资料所模拟的平均月度和平均年度补给量类似。然而,10年移动平均补给结果显示,观测的和气候资料模拟的气候资料有很大差别,特别是在1970–2000期间,采用观测的气候资料获取的结果有更大的变化性。ResumoNa avaliação de potenciais impactos da mudança climática nos recursos hídricos, a gestão de recursos hídricos busca entender como as condições futuras podem diferir do passado recente. Estudos sobre o impacto climático na recarga das águas subterrâneas geralmente comparam a recarga simulada de períodos futuros e históricos com base em uma média mensal ou média anual, ou camparam a recarga média de décadas futuras com aquela de uma única década recente. Estimativas da recarga histórica de referência, que são comparadas com condições futuras, são geralmente de simulação usando dados climáticos históricos observados. A comparação de resultados de médias mensais, resultados de médias anuais ou ainda calculando a média sobre décadas históricas selecionadas, podem mascarar a real variabilidade nos resultados históricos e levar a interpretação equivocada das condições futuras. A comparação resultados simulados da recarga futura usando dados climáticos de modelo de circulação geral (MCG) com resultados simulados da recarga usando dados climáticos históricos atuais também podem resultar em um entendimento incompleto das chances de futuras mudanças. Nesse estudo, a recarga das águas subterrâneas é estimada na bacia superior do Rio Colorado, EUA, usando o modelo de recarga das águas subterrâneas de balanço solo-água com parâmetros distribuídos para o período 1951–2000. Simulações da recarga foram feitas usando dados de precipitação, temperatura máxima e temperatura mínima a partir de dados climáticos observados e das projeções 97 CMIP5 (Coupled Model Intercomparison Project, phase 5). Os resultados indicam que a recarga média simulada mensalmente e anualmente são similares usando dados climáticos observados e do MCG. Entretanto, os resultados da recarga com uma média móvel de 10 anos mostraram diferenças substanciais entre os dados climáticos observados e simulados, particularmente durante o período 1970–2000, com variabilidade visível muito maior para os resultados usando dados climáticos observados.

Human effects on the hydrologic system of the Verde Valley, central Arizona, 1910–2005 and 2005–2110, using a regional groundwater flow model

Water budgets were developed for the Verde Valley of central Arizona in order to evaluate the degree to which human stresses have affected the hydrologic system and might affect it in the future. The Verde Valley is a portion of central Arizona wherein concerns have been raised about water availability, particularly perennial base flow of the Verde River. The Northern Arizona Regional Groundwater Flow Model (NARGFM) was used to generate the water budgets and was run in several configurations for the 1910–2005 and 2005–2110 time periods. The resultant water budgets were subtracted from one another in order to quantify the relative changes that were attributable solely to human stresses; human stresses included groundwater withdrawals and incidental and artificial recharge but did not include, for example, human effects on the global climate. Three hypothetical and varied conditions of human stresses were developed and applied to the model for the 2005–2110 period. On the basis of this analysis, human stresses during 1910–2005 were found to have already affected the hydrologic system of the Verde Valley, and human stresses will continue to affect the hydrologic system during 2005–2110. Riparian evapotranspiration decreased and underflow into the Verde Valley increased because of human stresses, and net groundwater discharge to the Verde River in the Verde Valley decreased for the 1910–2005 model runs. The model also showed that base flow at the upstream end of the study area, as of 2005, was about 4,900 acre-feet per year less than it would have been in the absence of human stresses. At the downstream end of the Verde Valley, base flow had been reduced by about 10,000 acre-feet per year by the year 2005 because of human stresses. For the 2005–2110 period, the model showed that base flow at the downstream end of the Verde Valley may decrease by an additional 5,400 to 8,600 acre-feet per year because of past, ongoing, and hypothetical future human stresses. The process known as capture (or streamflow depletion caused by the pumping of groundwater) was the reason for these human-stress-induced changes in water-budget components.

Changes in projected spatial and seasonal groundwater recharge in the upper Colorado River Basin

The Colorado River is an important source of water in the western United States, supplying the needs of more than 38 million people in the United States and Mexico. Groundwater discharge to streams has been shown to be a critical component of streamflow in the Upper Colorado River Basin (UCRB), particularly during low-flow periods. Understanding impacts on groundwater in the basin from projected climate change will assist water managers in the region in planning for potential changes in the river and groundwater system. A previous study on changes in basin-wide groundwater recharge in the UCRB under projected climate change found substantial increases in temperature, moderate increases in precipitation, and mostly periods of stable or slight increases in simulated groundwater recharge through 2099. This study quantifies projected spatial and seasonal changes in groundwater recharge within the UCRB from recent historical (1950 to 2015) through future (2016 to 2099) time periods, using a distributed-parameter groundwater recharge model with downscaled climate data from 97 Coupled Model Intercomparison Project Phase 5 (CMIP5) climate projections. Simulation results indicate that projected increases in basin-wide recharge of up to 15% are not distributed uniformly within the basin or throughout the year. Northernmost subregions within the UCRB are projected an increase in groundwater recharge, while recharge in other mainly southern subregions will decline. Seasonal changes in recharge also are projected within the UCRB, with decreases of 50% or more in summer months and increases of 50% or more in winter months for all subregions, and increases of 10% or more in spring months for many subregions.

The Effect of modeled recharge distribution on simulated groundwater availability and capture

Simulating groundwater flow in basin-fill aquifers of the semiarid southwestern United States commonly requires decisions about how to distribute aquifer recharge. Precipitation can recharge basin-fill aquifers by direct infiltration and transport through faults and fractures in the high-elevation areas, by flowing overland through high-elevation areas to infiltrate at basin-fill margins along mountain fronts, by flowing overland to infiltrate along ephemeral channels that often traverse basins in the area, or by some combination of these processes. The importance of accurately simulating recharge distributions is a current topic of discussion among hydrologists and water managers in the region, but no comparative study has been performed to analyze the effects of different recharge distributions on groundwater simulations. This study investigates the importance of the distribution of aquifer recharge in simulating regional groundwater flow in basin-fill aquifers by calibrating a groundwater-flow model to four different recharge distributions, all with the same total amount of recharge. Similarities are seen in results from steady-state models for optimized hydraulic conductivity values, fit of simulated to observed hydraulic heads, and composite scaled sensitivities of conductivity parameter zones. Transient simulations with hypothetical storage properties and pumping rates produce similar capture rates and storage change results, but differences are noted in the rate of drawdown at some well locations owing to the differences in optimized hydraulic conductivity. Depending on whether the purpose of the groundwater model is to simulate changes in groundwater levels or changes in storage and capture, the distribution of aquifer recharge may or may not be of primary importance.

Preliminary groundwater flow model of the basin-fill aquifers in Detrital, Hualapai, and Sacramento Valleys, Mohave County, northwestern Arizona

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