Abstract
AbstractGroundwater is an important component of the hydrological cycle with significant interactions with soil hydrological processes. Recent studies have demonstrated that incorporating groundwater hydrology in land surface models (LSMs) considerably improves the prediction of the partitioning of water components (e.g., runoff and evapotranspiration) at the land surface. However, the Joint UK Land Environment Simulator (JULES), an LSM developed in the United Kingdom, does not yet have an explicit representation of groundwater. We propose an implementation of a simplified groundwater flow boundary parameterization (JULES‐GFB), which replaces the original free drainage assumption in the default model (JULES‐FD). We tested the two approaches under a controlled environment for various soil types using two synthetic experiments: (1) single‐column and (2) tilted‐V catchment, using a three‐dimensional (3‐D) hydrological model (ParFlow) as a benchmark for JULES’ performance. In addition, we applied our new JULES‐GFB model to a regional domain in the UK, where groundwater is the key element for runoff generation. In the single‐column infiltration experiment, JULES‐GFB showed improved soil moisture dynamics in comparison with JULES‐FD, for almost all soil types (except coarse soils) under a variety of initial water table depths. In the tilted‐V catchment experiment, JULES‐GFB successfully represented the dynamics and the magnitude of saturated and unsaturated storage against the benchmark. The lateral water flow produced by JULES‐GFB was about 50% of what was produced by the benchmark, while JULES‐FD completely ignores this process. In the regional domain application, the Kling‐Gupta efficiency (KGE) for the total runoff simulation showed an average improvement from 0.25 for JULES‐FD to 0.75 for JULES‐GFB. The mean bias of actual evapotranspiration relative to the Global Land Evaporation Amsterdam Model (GLEAM) product was improved from −0.22 to −0.01 mm day−1. Our new JULES‐GFB implementation provides an opportunity to better understand the interactions between the subsurface and land surface processes that are dominated by groundwater hydrology.
Highlights
It is widely known that groundwater (GW) is of paramount importance for water management, as it represents 97% of available freshwater resources worldwide (Guppy et al, 2018)
For the two cases where the water table is initialized within the soil domain of Joint UK Land Environment Simulator (JULES), JULES-FD is unable to retain the soil moisture, resulting in much drier soil moisture profiles when compared to the Benchmark Model
Unlike the JULES-FD results, the new JULES-groundwater flow boundary (GFB) shows remarkably good agreement with the Benchmark Model in both shallow water table cases shown in Figure 5, except for some minor differences close to the surface that are related to the different water partitioning in ParFlow, compared to the two JULES models
Summary
It is widely known that groundwater (GW) is of paramount importance for water management, as it represents 97% of available freshwater resources worldwide (Guppy et al, 2018). Groundwater is the last critical national resource during droughts (Famiglietti et al, 2011), it will be key in future water management, knowing that climate change will likely increase the frequency of drought (e.g., Lehner et al, 2006). By representing groundwater in large-scale models, we can understand and quantify the interactions between groundwater and climate, understand and quantify the two-way interactions between the subsurface with the surface and the atmosphere and support decision making in transboundary groundwater systems (Gleeson et al, 2019). Groundwater representation is still neglected in most LSMs, it is crucial to incorporate such processes in order to improve the predictions of these models
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