Coupled Surface Subsurface Research Articles

Calibration guide for watershed modeling with distributed groundwater modeling: application for the SWAT+ model

ABSTRACT We present a new calibration strategy guide for coupled surface–subsurface hydrologic models. The goal is to reduce bias in environmental modeling and make calibration processes more consistent. The strategy guide enhances model efficiency and accuracy by focusing on changes to channel, soil, landscape, and aquifer properties. This approach is particularly beneficial when simulating important hydrologic features like baseflow and peaks and across different watersheds. Developed using the Soil and Water Assessment Tool (SWAT+) and the new gwflow module in six diverse US watersheds, each with unique hydrologic characteristics, the guide encourages improved calibration through routine use. It addresses specific issues related to the temporal distribution of streamflow and offers a robust method for enhancing model performance across a wide range of hydrologic conditions. By employing parameter estimation and sensitivity analysis, the approach ensures rigorous and objective calibration of SWAT+ and other models that include land, soil, aquifer, and water processes, leading to more accurate hydrologic modeling.

Assessing the informativeness of a coupled surface–subsurface watershed model for understanding debris flow: a hydrological perspective

ABSTRACT Characterization of debris flow is critical to both risk assessment and hazard mitigation. Recent technologies enable onsite environmental monitoring sensors for geological disaster monitoring. However, the spatiotemporal understanding of debris flow in remote mountainous areas is still limited due to difficulties in observation networks and its complex driving conditions. Here we apply a coupled surface–subsurface hydrological model to examine the characteristics of the water movements near debris flow sites in southwest China. Our approach captured the temporal dynamics of infiltration, redistribution of soil moisture, groundwater storage, and lateral groundwater fluxes. The lateral groundwater flux and groundwater storage were informative indicators in identifying debris flow location. Such informativeness was only effective when hourly dynamics were analysed. Our findings provide new insight into quantifying debris flow susceptibility. This study suggests that the coupled surface–subsurface watershed modelling approach can be informative for preliminary monitoring, planning and management of debris flow.

Rough ice cover modified hyporheic exchange: A numerical study on the mechanical effect propagating from the ice-water interface to the sediment–water interface

Many rivers are subject to ice-covered flow conditions in cold climate regions or during the winter months. The presence of ice cover can change stream flow hydrodynamics and thus affect the hyporheic exchange involved in many biogeochemical processes in aquatic environments. Among the various features of ice cover, its underside roughness is a main factor modifying the surface–subsurface flow hydrodynamics. To date, there has been little research on how ice cover affects the hyporheic exchange, even less on the effect of the ice cover roughness. This leaves open questions on the mechanical changes initiated at the ice-water interface and transferred to the water–sediment interface. In this study, a coupled surface–subsurface computational approach was used to model conjunctively the channel flow over dune bed and the underlying porous flow under different ice cover conditions. The results show that ice cover can enhance the hyporheic flux and reduce the hyporheic zone depth when compared to open channel flow at equivalent discharge. As the river flow depth increases to 4 times the height of the dune, the effect of the ice cover on the hyporheic flow becomes negligible. Within this effective range, the hyporheic flux via the channel water depth follows a power-law function. Ice cover roughness greatly augments the exchange rate while compresses the exchange space, therefore reduces flow residence time in the hyporheic zone. Based on the modeling results, prediction models were proposed for evaluating the impacts of ice cover roughness on the hyporheic exchange flux and depth. Our study indicates that hyporheic flow exchange is significantly impacted by the roughness of the ice cover which in turn the hyporheic exchange is likely to affect the river ice processes as well as the water quality and ecological functions throughout the river corridors.

On the development and analysis of coupled surface–subsurface models of catchments. Part 3. Analytical solutions and scaling laws

The objective of this three-part work is to formulate and rigorously analyse a number of reduced mathematical models that are nevertheless capable of describing the hydrology at the scale of a river basin (i.e. catchment). Coupled surface and subsurface flows are considered. In this third part, we focus on the development of analytical solutions and scaling laws for a benchmark catchment model that models the river flow (runoff) generated during a single rainfall. We demonstrate that for catchments characterised by a shallow impenetrable bedrock, the shallow-water approximation allows a reduction of the governing formulation to a coupled system of one-dimensional time-dependent equations for the surface and subsurface flows. Asymptotic analysis is used to derive semi-analytical solutions for the model. We provide simple asymptotic scaling laws describing the peak flow formation, and demonstrate its accuracy through a comparison with the two-dimensional model developed in Part 2. These scaling laws can be used as an analytical benchmark for assessing the validity of other physical, conceptual or statistical models of catchments.

On the development and analysis of coupled surface–subsurface models of catchments. Part 2. A three-dimensional benchmark model and its properties

The objective of this three-part work is to formulate and rigorously analyse a number of reduced mathematical models that are nevertheless capable of describing the hydrology at the scale of a river basin (i.e. catchment). Coupled surface and subsurface flows are considered. In this second part, we construct a benchmark catchment scenario and investigate the effects of parameters within their typical ranges. Previous research on coupled surface–subsurface models have focused on numerical simulations of site-specific catchments. Here, our focus is broad, emphasising the study of general solutions to the mathematical models, and their dependencies on dimensionless parameters. This study provides a foundation based on the examination of a geometrically simple three-dimensional benchmark scenario. We develop a non-dimensional coupled surface–subsurface model and extract the key dimensionless parameters. Asymptotic methods demonstrate under what conditions the model can be reduced to a two-dimensional form, where the principal groundwater and overland flows occur in the hillslope direction. Numerical solutions provide guidance on the validity of such reductions, and demonstrate the parametric dependencies corresponding to a strong rainfall event.

On the development and analysis of coupled surface–subsurface models of catchments. Part 1. Analysis of dimensions and parameters for UK catchments

The objective of this three-part work is to formulate and rigorously analyse a number of reduced mathematical models that are nevertheless capable of describing the hydrology at the scale of a river basin (i.e. catchment). Coupled surface and subsurface flows are considered. In this first part, we identify and analyse the key physical parameters that appear in the governing formulations used within hydrodynamic rainfall–runoff models. Such parameters include those related to catchment dimensions, topography, soil and rock properties, rainfall intensities, Manning's coefficients and river channel dimensions. Despite the abundance of research that has produced data sets describing properties of specific river basins, there have been few studies that have investigated the ensemble of typical scaling of key physical properties; these estimates are needed to perform a proper dimensional analysis of rainfall–runoff models. Therefore, in this work, we perform an extensive analysis of the parameters; our results form a benchmark and provide guidance to practitioners on the typical parameter sizes and interdependencies. Crucially, the analysis is presented in a fashion that can be reproduced and extended by other researchers and, wherever possible, uses publicly available data sets for catchments in the UK.

Flow paths and wetness conditions explain spatiotemporal variation of nitrogen retention for a temperate, humid catchment

Excess use of nitrogen (N) fertilizer has threatened the quality of drinking water and destroyed the structure and functions of aquatic ecosystems. The N retention is defined as a catchment’s capacity to withhold nitrogen from entering surface water bodies during a certain period. Previous studies have focused on N retention in response to catchment characteristics and different climates. However, little has been done to explore the spatiotemporal variation of N retention within a catchment. This study is aimed to explore the spatiotemporal variation of the N retention and the effect of the external N input heterogeneity on the retention capacity and the N export in a small temperate, humid catchment in central Germany. A spatially distributed model was built using the fully coupled surface–subsurface model HydroGeoSphere to simulate the flow and transport of nitrate. Three scenarios considering the external N input heterogeneity, which represent different land-use patterns, were created: (i) the input was uniformly distributed over the catchment, (ii) high input was associated with high retention regions (e.g. near the hillslope top), and (iii) high input was associated with low retention regions (e.g. near the stream). The results showed that N retention exhibits strong spatial heterogeneity, which is controlled by the lengths of flow paths connecting the regions and surface water body and regional wetness conditions within a catchment. N retention showed pronounced seasonality in temperate and humid climates, being higher in wetting, drying and dry periods, but lower in wet periods. The pattern is driven by distinct shifts in the dominance of deeper flow paths and fast shallow flow paths determined by the climate seasonality. Highest denitrification, lowest export fluxes and in-stream nitrate concentration were found in scenario (ii) among the scenarios. Therefore, it was suggested that agricultural activities may avoid regions near surface water bodies to improve the surface water quality. The results of this study provide insights into the nitrogen dynamics and can guide the management of surface water quality.

A process-based hydrological model for continuous multi-year simulations of large-scale watersheds

Process-based hydrological models are useful tools to understand the mechanisms responsible for extreme floods and droughts and thus facilitate watershed management. This paper describes a watershed model, based on the coupled surface-subsurface distributed model Penn State Hydrologic Model (PIHM), to capture the hydrologic response of large watersheds over decades. Mathematical models for coupling surface-subsurface fluxes were developed to achieve mass conservation in multi-year simulations. A Voronoi technique was implemented to improve accuracy and computational efficiency. The model was used to simulate the Cedar River Watershed, IA from 2003 to 2018. Model predictions were compared against streamflow observations at four gauging stations. Results show good model performance with average Nash–Sutcliffe efficiency values greater than 0.70. The model results helped to identify the main hydrologic processes that contributed to the historic 2008 flood in the Cedar River Watershed. In addition to heavy storms, several other factors played an important role, including snowmelt and rainstorms in the spring, as well as low evapotranspiration values associated with traditional row crop agriculture in the first half of the year.

3D surface–subsurface modeling of a bromide tracer test in a macroporous tile‐drained field: Improvements and limitations

AbstractThe assessment and forecasting of nutrient loss by tile drains in agricultural areas often rely on physically based models that have adequate representations of macropores and tile drains. Macroporosity has been adequately represented in hydrological models using a dual continuum approach. However, its implementation in hydrological and solute transport models is limited to plot‐scale or to one‐ and two‐dimensional models due to the large number of parameters that are rarely available and the long computational times. The purpose of this study is to simulate a tracer test using a 3D coupled surface–subsurface model to improve the representation of the tracer concentration at the drainage discharge. A three‐dimensional HydroGeoSphere model was developed and calibrated to simulate tile drainage discharge, Br mass discharge, and hydraulic heads from a Br tracer test in a densely tile‐drained field. The conductivity of the drain was one of the most important parameters for drain discharge and solute transport simulations. The model accurately simulated drainage discharge and Br transport to tile drains. However, most of the Br peaks and the late‐time Br mass in the drain outflow were underestimated. Our simulation results indicate that explicitly representing tile drains with seepage nodes allows for a physically based, yet computationally efficient representation of Br transport behavior surrounding tile drains at field scale. However, we cannot confirm that the single‐porosity model with immobile zone is suitable for simulating the Br peaks at the drain outlet and the late‐time Br mass. Improvements to the model include the implementation of heterogeneous soil layers and the inclusion of more measured data to reduce uncertainty during calibration.

Regional coupled surface–subsurface hydrological model fitting based on a spatially distributed minimalist reduction of frequency domain discharge data

Abstract. Although integrated water resource models are indispensable tools for water management at various scales, it is of primary importance to ensure their proper fitting on hydrological variables, avoiding flaws related to equifinality. An innovative stepwise fitting methodology is therefore proposed, which can be applied for any river basin model, from catchment to continental scale as far as hydrological models or land surface models are concerned. The methodology focuses on hydrosystems considering both surface water and groundwater, as well as internal water fluxes such as river baseflow. It is based on the thorough analysis of hydrological signal transformation by various components of a coupled surface–subsurface hydrosystem in a nested approach that considers the conditionality of parameter fields on their input forcing fluxes. The methodology is based on the decomposition of hydrological signal in the frequency domain with the HYMIT (HYdrological MInimalist Transfer function) method (Schuite et al., 2019). Parameters derived from HYMIT are used to fit the coupled surface–subsurface hydrological model CaWaQS3.02 using a stepwise methodology, which relies on successive Markov chain Monte Carlo optimizations related to various objective functions representing the dependency of the hydrological parameter fields on forcing input fluxes. This new methodology enables significant progress to be made in terms of the spatial distribution of the model parameters and the water balance components at the regional scale. The use of many control stations such as discharge gauging stations with HYMIT leads to a coarse parameter distribution that is then refined by the fitting of CaWaQS parameters on its own mesh. The stepwise methodology is exemplified with the Seine River basin (∼76 000 km2). In particular, it made it possible to spatially identify fundamental hydrological values, such as rainfall partitioning into actual evapotranspiration, as well as runoff and aquifer recharge through its impluvium, in both the time and frequency domains. Such a fitted model facilitates the analysis of both the overall and detailed territorial functioning of the river basin, explicitly including the aquifer system. A reference piezometric map of the upmost free aquifer units and a water budget of the Seine basin are established, detailing all external and internal fluxes up to the exchanges between the eight simulated aquifer layers. The results showed that the overall contribution of the aquifer system to the river discharge of the river network in the Seine basin varies spatially within a wide range (5 %–96 %), with an overall contribution at the outlet of the basin of 67 %. The geological substratum greatly influences the contribution of groundwater to the river discharge.

Impacts of sea-level rise on groundwater inundation and river floods under changing climate

Rising sea levels increase coastal extreme events of inundation along the global coastlines. Existing hydrologic and groundwater models simulate inundation processes but simplify groundwater-surface water interactions, a key control on flood generation in coastal areas. Here we apply a coupled surface–subsurface hydrologic model to evaluate the sensitivity of shallow groundwater to sea-level rise across three watersheds in Southeast China. It was found that historical decrease of annual rainfall resulted in no changes in total water storage but lower streamflow. Sea-level rise can increase low flow and local groundwater inundation, without changes in total water storage. GIS analysis of elevation can give a first approximation of vulnerability to marine inundation. Our physics-based modeling approach helped capture the temporal dynamics of water table and improved process understanding on coastal flooding. This study provides a new insight to quantify the spatiotemporal dynamics of groundwater inundation and river floods across coastal watersheds.

Impact of stormwater infiltration on rainfall-derived inflow and infiltration: A physically based surface–subsurface urban hydrologic model

The subsurface fate of stormwater infiltration enhanced by green infrastructure (GI) is unknown. GI redirects stormflows from the surface into the subsurface, which can be partitioned to evapotranspiration or baseflow, or be intercepted by buried infrastructure trenches. Stormwater that infiltrates sanitary sewer pipes is referred to as rainfall-derived inflow and infiltration (RDII) and represents a rapid pathway for infiltrated stormwater to reach receiving waters. To quantify the partitioning of infiltrated stormwater between slow and fast flows, a physically-based hydrologic model was developed integrating coupled surface–subsurface processes and urban sewer systems and applied to a sewershed near Milwaukee, WI. In this region with shallow groundwater table and leaky sanitary sewer systems, RDII accounted for 73–79% of the sanitary sewer flow volume and 21% of the sewershed water balance. Sensitivity analysis indicated that the surface direct inflow was controlled more by precipitation/evapotranspiration ratio and the subsurface infiltration was controlled more by groundwater table depth and sanitary sewer defect density. The largest effect of GI was to shift surface runoff to evapotranspiration and reduce peak flow in urban sewer systems. The volume of RDII was relatively less sensitive to GI infiltration because reductions in directly connected surface runoff were offset by increases in groundwater inflow. A moderate level of GI implementation (5–10% replacement of impervious area) reduced RDII volume by 10%, however, these changes were less than 1% of the overall water balance. On the other hand, higher levels of GI implementation (∼20% replacement of impervious area) resulted in no change to RDII volume as more infiltrated stormwater was routed to the sewer system through the subsurface. This study highlights the necessity of considering the full hydrologic context of GI and balancing the runoff reduction by GI and groundwater infiltration into sewers.

On the intrusion and recovery of ocean-sourced 3H and 14C in coastal aquifers considering ocean-surge inundation

Human activities, such as discharging nuclear wastewater into the ocean, can put coasts in the risk of being exposed to radioactive contaminations. The radionuclides may potentially intrude into coastal aquifers from the ocean and endanger the groundwater resources, either laterally through subsurface or vertically through land surface during ocean surge induced inundation. This study attempted to investigate the temporal and spatial scales of these intrusion processes using an experimental site near Bremerhaven, northern Germany. A fully coupled surface–subsurface flow and transport model was employed to simulate the groundwater flow and the transport of TDS (total dissolved salt), 3H and 14C, under conditions with or without an ocean surge event. Scenarios of different exposure schemes were designed by varying the exposure duration and radionuclide concentrations. The intrusion and post-exposure recovery were assessed via contaminated volume and area, intrusion length, and recovery time. Results show that, depending on exposure duration, the intrusion of 3H and 14C can cause serious contamination in the coastal aquifer, with up to 18.9 km2 land area being contaminated and reaching up to 3.3 km landward from the shore. 3H shows a ‘harder intrusion, easier recovery’ pattern due to its higher decay rate. The full recovery may take up to 70 years and 217 years (considering 30-years-exposure) for 3H and 14C, respectively. An ocean surge event is more likely to exacerbate the intrusion, but it does not significantly change the full recovery time. Shorter exposure duration should be favored to minimize the influences on the coastal aquifer when the total exposure quantity is fixed. This study highlights the severity of radioactive contamination in coastal aquifers and the difficulty of recovery. Thus, cautious decision should be made concerning the possible impacts of releasing radioactive wastewater on coastal life and human beings.

Effect of cut-off wall on freshwater storage in small islands considering ocean surge inundation

Freshwater lenses (FWL) underneath islands are often unique freshwater resources. Constructing cut-off walls in coastal islands to increase the FWL size has been proposed and tested effective. Meanwhile, seawater inundation induced by ocean surges on low-lying islands can cause the vertical intrusion of saltwater and salinization of FWL. Constructing cut-off walls can influence the post-inundation recovery once the FWL is salinized during the inundation. This influence has not been investigated and considered when discussing the feasibility of cut-off walls. In this study, we numerically simulated an ocean-surge inundation on a 2D, vertically oriented island constructed with a cut-off wall, using a coupled surface–subsurface approach. We varied the aquifer permeability and the depth of the cut-off wall. The effect of the cut-off wall on FWL was quantitatively assessed in terms of FWL volume and flushing time. The results show that the cut-off wall can significantly increase the volume of FWL, confirming previous studies, especially when it is constructed in high-permeability (>10−10.5 m2) islands and deeper. In addition, the cut-off wall can significantly reduce seawater infiltration by up to 40% during the inundation. Therefore, the recovery tends to be accelerated in the early stage, and in islands of low & intermediate permeability. However, the average recovery in high-permeability islands can be slowed down, because the cut-off wall reclaims larger and deeper space that is harder for flushing. As a first attempt to assess the feasibility of the cut-off wall construction from the perspective of FWL recovery, our study is important for the management of freshwater on islands facing the risk of ocean-surge inundation.

Numerical dispersion of solute transport in an integrated surface–subsurface hydrological model

Integrated surface–subsurface hydrological models (ISSHMs) are increasingly being used for the assessment of contaminant transport in the environment, in addition to their more common use in water flow applications. However, the subsurface solute transport solvers in these models are prone to numerical dispersion errors. Numerical dispersion is a well-known issue in groundwater modeling, but its impacts on the results of ISSHM simulations are still poorly understood. In this study, the CATchment HYdrology (CATHY) model is used to assess the potential impacts of numerical dispersion on the simulation of coupled surface–subsurface solute transport. We first simulate the subsurface transport of a nonreactive tracer in two soil column test cases (1D and 3D) with known analytical solutions. The subsurface solute transport solver in CATHY adopts a computationally efficient time-splitting technique whereby the advection component of the governing equation is solved on elements and the hydrodynamic dispersion component is solved on nodes. Comparison between simulation results and analytical solutions with different mesh discretizations and different values for the hydrodynamic dispersion coefficients allows for accurate quantification of the numerical dispersion error and yields insights into the parameters and other factors that control it. It is shown that, taken alone, the advection and dispersion solvers are very robust, but their combination can result in significant numerical dispersion, stemming from the exchange of concentration information from elements to nodes and vice versa in the time-splitting procedure. The tests also show that these errors can be kept under control by ensuring that the grid Péclet number is in the range 0.5-1.0 or smaller. We then apply CATHY in a third test case involving two synthetic hillslopes (concave and convex) in fully coupled surface–subsurface mode, in order to examine the impact of this subsurface numerical dispersion on simulated streamflow hydrographs, in particular with reference to pre-event water contributions to runoff. Here as well the results show that the effect of numerical dispersion can be controlled by keeping the grid Péclet number sufficiently small. This work provides a new set of benchmark test cases for integrated surface–subsurface hydrological models, extending to solute transport the flow-only suite of benchmarks recently published in two intercomparison studies.

Capturing hotspots of fresh submarine groundwater discharge using a coupled surface–subsurface model

Submarine groundwater discharge (SGD) contributes to the physical and chemical characters of coastal waters by discharging nutrients and contaminants, significantly impacting regional marine ecosystems and contributing to ocean chemical budgets. However, such groundwater discharge varies dramatically across scales and is often not comparable due to different model assumptions and field designs. We used a hydrologic model with integration of fundamental surface and subsurface processes to simulate the coastline level fresh SGD for the Crete Island in the Mediterranean Sea. The modeled hydrological processes suggested that fresh SGD substantially contributes to water flow entering the Mediterranean Sea (2.3 × 108 m3/yr), amounting to 31% of river discharge and 14% of precipitation. Spatially, fresh SGD varied from 2.4 m3/yr/m to 13.4 × 104 m3/yr/m, with an average of 2.6 × 103 m3/yr/m. The local maxima were commonly associated with river mouths reflecting larger hydraulic gradients and higher permeable structures. Temporally, fresh SGD was impacted by episodic precipitation in a delayed and prolonged pattern. We found that fresh SGD variability at the coastline segment level was compared to point measurements and fresh SGD magnitudes summered up to the catchment level were consistent with global products. Our results suggest the coupled surface–subsurface hydrologic modeling approach is a promising strategy to quantify and partition large-scale water budgets down to point observations that typically do not capture the full range of fresh SGD dynamics.

Coupled Surface–Subsurface Flow Modelling Using the GeRa Software

Mathematical model and numerical methods for the solution of coupled surface–subsurface flow in GeRa software is presented. Coupling of surface runoff and subsurface flow models is accomplished using first-order exchange term. The arising nonlinear solver convergence issues are addressed in the article. In order to overcome these issues, approaches for handling the surface–subsurface flux expression are introduced. The proposed numerical model is verified and validated using benchmarks with reference solutions obtained in different codes as well as experimental results.

Simulating coupled surface–subsurface flows with ParFlow v3.5.0: capabilities, applications, and ongoing development of an open-source, massively parallel, integrated hydrologic model

Abstract. Surface flow and subsurface flow constitute a naturally linked hydrologic continuum that has not traditionally been simulated in an integrated fashion. Recognizing the interactions between these systems has encouraged the development of integrated hydrologic models (IHMs) capable of treating surface and subsurface systems as a single integrated resource. IHMs are dynamically evolving with improvements in technology, and the extent of their current capabilities are often only known to the developers and not general users. This article provides an overview of the core functionality, capability, applications, and ongoing development of one open-source IHM, ParFlow. ParFlow is a parallel, integrated, hydrologic model that simulates surface and subsurface flows. ParFlow solves the Richards equation for three-dimensional variably saturated groundwater flow and the two-dimensional kinematic wave approximation of the shallow water equations for overland flow. The model employs a conservative centered finite-difference scheme and a conservative finite-volume method for subsurface flow and transport, respectively. ParFlow uses multigrid-preconditioned Krylov and Newton–Krylov methods to solve the linear and nonlinear systems within each time step of the flow simulations. The code has demonstrated very efficient parallel solution capabilities. ParFlow has been coupled to geochemical reaction, land surface (e.g., the Common Land Model), and atmospheric models to study the interactions among the subsurface, land surface, and atmosphere systems across different spatial scales. This overview focuses on the current capabilities of the code, the core simulation engine, and the primary couplings of the subsurface model to other codes, taking a high-level perspective.

A 2D curvilinear coupled surface–subsurface flow model for simulation of basin/border irrigation: theory, validation and application

Despite the development of pressurized irrigation systems in the second half of the twentieth century, surface irrigation continues to be the most used system in the world. Computer tools are required to support performance improvements leading to its sustainability. A simulation model of basin/border irrigation is presented in this paper combining two-dimensional overland hydraulics based on Saint–Venant equations with three-dimensional infiltration based on the mixed form of Richards’ equation. The coupling of these equations is attained by enforcing continuity of pressure at the soil surface. The model applies a finite-volume approach using the diffusion-wave approximation form of the shallow water equations. Water flow on undulated topographies is simulated using a non-orthogonal curvilinear coordinate system. An underrelaxed-modified Picard iteration algorithm is used for Richards’ equation, and an underrelaxed Picard iteration algorithm is used for Saint–Venant equations. The model was validated using experiments from the literature. Model performance was robust and accurate, even in complex theoretical cases with strong soil-surface undulations. Mass conservation was judged satisfactory in all cases, with the long-term ratio of mass balance error in the order of 10−2 in the most complex simulations. Soil type, mesh non-orthogonality, and the interaction of these variables had a strong effect on CPU time. Additional research will be required to transform this simulation model into an operational tool for surface irrigation. Attention will need to be paid to additional surface irrigation systems, soil types, and the optimization of computational speed.

Moving beyond run‐off calibration—Multivariable optimization of a surface–subsurface–atmosphere model

AbstractSpatially distributed hydrological models are traditionally calibrated and evaluated against few spatially aggregated observations such as river discharge. This model evaluation approach does not enable an assessment of the model predictive capabilities of other hydrological states and fluxes nor does it give any insight into the model ability to mimic the spatial patterns within a catchment. The current study explores a multivariable optimization of a complex coupled surface–subsurface–atmosphere model at the catchment scale in an attempt to move beyond simple run‐off calibration. The model is evaluated against five independent observational data sets of discharge (Q), hydraulic head (h), actual evapotranspiration (ET), soil moisture (SM), and remotely sensed land surface temperature (LST). It is shown that a balanced optimization can be achieved where errors on objective functions for all five observation data sets can be reduced simultaneously. Additionally, the multivariable calibration proved more robust, compared with calibration against Q and h only, during the validation period, even for Q and h. The current parameterization and calibration framework was mainly suitable for reducing model biases and allowed only limited improvements in the spatio‐temporal patterns of the model simulations. This points towards development of better parametrization schemes that will allow simulated spatial patterns to adjust during calibration. Additionally, analysis showed that systematic spatial patterns in the errors of the LST maps could be a very valuable diagnostic tool for assessing deficiencies in the model structure, spatial parameterization, or process description.