Although hydrologists have worked on water quantity and quality problems in local watersheds for a century, their data/knowledge has not been fully tapped to improve the representation of hydrologic processes in Earth System models (ESMs). This disconnect is critical, because mountain watersheds exert outsize influence on seasonal and annual streamflow and, therefore, water availability for irrigation, industry, and human consumption. More generally, because water is essential to life, knowing how much water we have where and when, and how that will change in the future, constitutes one of society’s most basic scientific quests.

Here, we describe progress to advance the representation of hydrology in ESMs through the representation of slope, aspect, and lateral water transfer through a representative hillslope concept. This effort stems from the well-known and established feature that hillslope slope and aspect are critical drivers of energy and water processes and that water is redistributed across a hillslope due to gradients in elevation and water table position. This redistributed water contributes to the presence of riparian zones where water can be plentiful and vegetation lush, even in relatively dry climates. To account for these key features of the terrestrial system, within the context of a large-scale model, we implemented the capacity to include one or more representative hillslopes, each of which is made up of multiple soil columns, into the naturally vegetated land unit of the Community Land Mode (CLM)l. The representative or conceptual hillslope model allows for a computationally efficient representation of within-grid groundwater flow and resultant impacts on evapotranspiration, vegetation, and carbon cycling. Here, we describe tools that allow users to generate representative hillslopes, globally, at any large-scale grid resolution. We also demonstrate the impact of representative hillslopes in global historical and projection period simulations and establish where and when hillslope processes, including lateral water redistribution, snow redistribution, slope, aspect and other terrain features exert strong or weak impacts on the simulation of various quantities such as runoff, ET, vegetation productivity and state, and carbon cycling.