Yucaipa Integrated Hydrologic Model: simulating future climate scenarios in the Yucaipa Valley Watershed, San Bernardino and Riverside Counties, California

Managing water resources in semiarid watersheds is challenging because of limited water supply and uncertain impacts of future climate conditions on groundwater resources. This paper examines the impact of future climate changes on the urban Yucaipa Valley watershed in southern California using an integrated hydrological model referred to herein as the Yucaipa model. Coupled Groundwater and Surface-water FLOW (GSFLOW) modeling software was used to simulate the nonlinear relationships between climate trends and precipitation partitioning into evapotranspiration (ET), runoff, and subsurface storage. Four global climate models (also known as general circulation models or GCMs), each with two greenhouse-gas (GHG) scenarios: Representative Concentration Pathway 4.5 (RCP45) (low emissions) and RCP85 (high emissions) are used to project varying future climate conditions. GCMs include the Canadian Earth System Model (CanESM2), Centre National de Recherches Météorologiques Climate Model version 5 (CNRM-CM5), Hadley Centre Global Environment Model version 2 – Earth System (HadGEM2-ES), and Model for Interdisciplinary Research on Climate version 5 (MIROC5) models. RCP85 scenarios tend to be wetter and warmer than RCP45, and in some cases the increased precipitation offsets increased loss to ET. The Yucaipa model's simulated hydrological conditions using climate projections predict decreased groundwater in storage in most scenarios due to increased natural ET, vegetation consumptive use, and streamflow out of the watershed. Only scenarios with substantial increases in annual precipitation were able to maintain groundwater in storage approximately the same as at the end of the historical period. The study also highlights increased future aridity despite increased precipitation, and larger precipitation events, which increase the risk of urban floods and decrease stream leakage.

半干旱流域的水资源管理颇具挑战，这源于供水有限且未来气候条件对地下水资源的影响存在不确定性。本文以加利福尼亚州南部的尤卡帕谷城市流域为研究对象，采用本文所提及的集成水文模型——尤卡帕模型（Yucaipa model），分析未来气候变化对该流域的影响。研究使用耦合地下水与地表水流动模型（GSFLOW），模拟气候趋势与降水分配至蒸散发（Evapotranspiration, ET）、径流及地下储水之间的非线性关系。本次研究选用4种全球气候模型（亦称大气环流模型，GCMs），每种模型对应两种温室气体（GHG）排放情景：典型浓度路径4.5（RCP45，低排放情景）与RCP85（高排放情景），以此预估不同的未来气候条件。所选用的GCMs包括加拿大地球系统模型（CanESM2）、法国国家气象研究中心气候模型第5版（CNRM-CM5）、哈德利中心全球环境模型第2版——地球系统（HadGEM2-ES）以及气候跨学科研究模型第5版（MIROC5）。RCP85情景相较于RCP45情景往往更湿润、更温暖，在部分情景中，降水量的增加能够抵消因ET上升带来的水分损失。基于气候预估的尤卡帕模型模拟水文结果显示，在多数情景下，由于自然ET上升、植被耗水量增加以及流域径流出流增多，地下储水量将出现下降。仅当年降水量大幅增加的情景，能够使地下储水量维持在与历史时期末期大致相当的水平。本研究同时指出，尽管降水量有所增加，但未来干旱程度仍会加剧，且极端降水事件增多，这会提升城市洪涝风险并降低河道渗漏量。