Conjunctive use of surface and groundwater in central Asia area: a case study of the Tailan River Basin

Conjunctive use of surface and groundwater is an effective approach to relief water resources shortages and its uneven temporal and spatial distribution in arid inland regions of central Asia. In this paper, 16 characteristic factors of water resources and related systems which are sensitive to different types of conjunctive use were selected in order to develop an index system for evaluation impact of conjunctive use, based on the fundamental types of water resources conjunctive use in inland basins. In an attempt to address the issue of spring drought and summer floods in Tailan River Basin in Xinjiang, three scenarios of conjunctive use of surface water and groundwater, well-canal combination, piedmont reservoirs and groundwater reservoirs were investigated. The utilization of water resources and the response of its related systems were evaluated and analyzed quantitatively using the groundwater numerical simulation model (Visual MODFLOW). Furthermore, the impacts of conjunctive use were assessed using the method of multi-level fuzzy comprehensive assessment. The results show that the scheme of combination of Laolongkou piedmont reservoir regulation-groundwater development, is the best in terms of the evaluation of water conjunctive use effects, followed by the well-canal combination. The comprehensive effect of well-canal combination along with groundwater reservoir wouldn’t be better than the former both.     

Assessment of Managed Aquifer Recharge Site Suitability Using a GIS and Modeling

We completed a two‐step regional analysis of a coastal groundwater basin to (1) assess regional suitability for managed aquifer recharge (MAR), and (2) quantify the relative impact of MAR activities on groundwater levels and sea water intrusion. The first step comprised an analysis of surface and subsurface hydrologic properties and conditions, using a geographic information system (GIS). Surface and subsurface data coverages were compiled, georeferenced, reclassified, and integrated (including novel approaches for combining related datasets) to derive a spatial distribution of MAR suitability values. In the second step, results from the GIS analysis were used with a regional groundwater model to assess the hydrologic impact of potential MAR placement and operating scenarios. For the region evaluated in this study, the Pajaro Valley Groundwater Basin, California, GIS results suggest that about 7% (15 km²) of the basin may be highly suitable for MAR. Modeling suggests that simulated MAR projects placed near the coast help to reduce sea water intrusion more rapidly, but these projects also result in increased groundwater flows to the ocean. In contrast, projects placed farther inland result in more long‐term reduction in sea water intrusion and less groundwater flowing to the ocean. This work shows how combined GIS analysis and modeling can assist with regional water supply planning, including evaluation of options for enhancing groundwater resources.     

Groundwater–surface water interactions in a lowland watershed: source contribution to stream flow

The lower coastal plain of the Southeast USA is undergoing rapid urbanisation as a result of population growth. Land use change has been shown to affect watershed hydrology by altering stream flow and, ultimately, impairing water quality and ecologic health. However, because few long‐term studies have focused on groundwater–surface water interactions in lowland watersheds, it is difficult to establish what the effect of development might be in the coastal plain region. The objective of this study was to use an innovative improvement to end‐member mixing analysis (EMMA) to identify time sequences of hydrologic processes affecting storm flow. Hydrologic and major ion chemical data from groundwater, soil water, precipitation and stream sites were collected over a 2‐year period at a watershed located in USDA Forest Service's Santee Experimental Forest near Charleston, South Carolina, USA. Stream flow was ephemeral and highly dependent on evapotranspiration rates and rainfall amount and intensity. Hydrograph separation for a series of storm events using EMMA allowed us to identify precipitation, riparian groundwater and streambed groundwater as main sources to stream flow, although source contribution varied as a function of antecedent soil moisture condition. Precipitation, as runoff, dominated stream flow during all storm events while riparian and streambed groundwater contributions varied and were mainly dependent on antecedent soil moisture condition. Sensitivity analyses examined the influence of 10% and 50% increases in analyte concentration on EMMA calculations and found that contribution estimates were very sensitive to changes in chemistry. This study has implications on the type of methodology used in traditional forms of EMMA research, particularly in the recognition and use of median end‐member water chemistry in hydrograph separation techniques. Potential effects of urban development on important hydrologic processes (groundwater recharge, interflow, runoff, etc.) that influence stream flow in these lowland watersheds were qualitatively examined. Copyright © 2011 John Wiley & Sons, Ltd.     

Modelling the potential impacts of groundwater hydrology on long‐term drainage basin evolution

Fluvial erosion processes are driven by water discharge on the land surface, which is produced by surface runoff and groundwater discharge. Although groundwater is often neglected in long‐term landscape evolution problems, water table levels control patterns of Dunne runoff production, and groundwater discharge can contribute significantly to storm flows. In this analysis, we investigate the role that groundwater movement plays in long‐term drainage basin evolution by modifying a widely used landscape evolution model to include a more detailed representation of basin hydrology. Precipitation is generated by a stochastic process, and the precipitation is partitioned between surface runoff and groundwater recharge using a specified infiltration capacity. Groundwater flow is simulated by a dynamic two‐dimensional Dupuit equation for an unconfined aquifer with an irregular underlying impervious layer. The model is applied to the WE‐38 basin, an experimental catchment in Pennsylvania, because 60–80 per cent of the discharge is derived from groundwater and substantial hydrologic and geomorphic information is available. The hydrologic model is first calibrated to match the observed streamflows, and then the combined hydrologic/geomorphic model is used to simulate scenarios with different infiltration capacities. The results of this modelling exercise indicate that the basin can be divided into three zones with distinct streamflow‐generating characteristics, and different parts of the basin can have different geomorphic effective events. Over long periods of time, scenarios in which groundwater discharge is large tend to modify the topography in a way that promotes groundwater discharge and inhibits Dunne runoff. Copyright © 2006 John Wiley & Sons, Ltd.     

IHMS—Integrated Hydrological Modelling System. Part 1. Hydrological processes and general structure

A newly Integrated Hydrological Modelling System (IHMS) has been developed to study the impact of changes in climate, land use and water management on groundwater and seawater intrusion (SWI) into coastal areas. The system represents the combination of three models, which can, if required, be run separately. It has been designed to assess the combined impact of climate, land use and groundwater abstraction changes on river, drainage and groundwater flows, groundwater levels and, where appropriate, SWI. The approach is interdisciplinary and reflects an integrated water management approach. The system comprises three packages: the Distributed Catchment Scale Model (DiCaSM), MODFLOW (96 and 2000) and SWI models. In addition to estimating all water balance components, DiCaSM, produces the recharge data that are used as input to the groundwater flow model of the US Geological Survey, MODFLOW. The latter subsequently generates the head distribution and groundwater flows that are used as input to the SWI model, SWI. Thus, any changes in land use, rainfall, water management, abstraction, etc. at the surface are first handled by DiCaSM, then by MODFLOW and finally by the SWI. The three models operate at different spatial and temporal scales and a facility (interface utilities between models) to aggregate/disaggregate input/output data to meet a desired spatial and temporal scale was developed allowing smooth and easy communication between the three models. As MODFLOW and SWI are published and in the public domain, this article focuses on DiCaSM, the newly developed unsaturated zone DiCaSM and equally important the interfacing utilities between the three models. DiCaSM simulates a number of hydrological processes: rainfall interception, evapotranspiration, surface runoff, infiltration, soil water movement in the root zone, plant water uptake, crop growth, stream flow and groundwater recharge. Input requirements include distributed data sets of rainfall, land use, soil types and digital terrain; climate data input can be either distributed or non‐distributed. The model produces distributed and time series output of all water balance components including potential evapotranspiration, actual evapotranspiration, rainfall interception, infiltration, plant water uptake, transpiration, soil water content, soil moisture (SM) deficit, groundwater recharge rate, stream flow and surface runoff. This article focuses on details of the hydrological processes and the various equations used in DiCaSM, as well as the nature of the interface to the MODFLOW and SWI models. Furthermore, the results of preliminary tests of DiCaSM are reported; these include tests related to the ability of the model to predict the SM content of surface and subsurface soil layers, as well as groundwater levels. The latter demonstrates how the groundwater recharge calculated from DiCaSM can be used as input into the groundwater model MODFLOW using aggregation and disaggregation algorithms (built into the interface utility). SWI has also been run successfully with hypothetical examples and was able to reproduce the results of some of the original examples of Bakker and Schaars ( 2005 ). In the subsequent articles, the results of applications to different catchments will be reported. Copyright © 2010 John Wiley & Sons, Ltd.     

Development of a Composite Hydrologic Index for Semi-Arid Region of India

Rainfall is the major source for groundwater recharge in basins areas of central region of India. Now a day, the river basins are experiencing acute shortage of water which has resulted in lowering of groundwater level and drying up of water bodies. In order to maintain water sustainability; a composite hydrologic index was developed in the Betwa basin of Madhya Pradesh and Uttar Pradesh states, India. The index was developed using principal component analysis through hydrologic, topographic as well as geographic parameters derived from the Soil and Water Assessment Tool and MODFLOW model. The geomorphological parameters were categorized, on the basis of groundwater recharge potential and weight ranged from 1 to 4. The geomorphologic parameters, that is, soil type (T), slope (S), runoff ratio (R), and evapotranspiration (ET) were integrated into a single indicator of composite hydrologic index. Soil type and ET were the major factors that directly affected the groundwater recharge. These two parameters together explained 86% of total variability in the data. Based on the analysis of the four parameters that affected groundwater recharge, composite hydrologic index (CHI) was classified into very good, good, moderate, and low grade. The CHI was statistically validated using standardization methods. The index was developed as a water management tool to measure a sustainability state relative to a groundwater recharge potential, which allows for spatial and temporal comparison. This index will be helpful in natural resource management and will improve socioeconomic status of human population inhibiting in the semi-arid region.     

The impact of groundwater–surface water interactions on the water balance of a mesoscale lowland river catchment in northeastern Germany

The glacially formed northeastern German lowlands are characterized by extensive floodplains, often interrupted by relatively steep moraine hills. The hydrological cycle of this area is governed by the tight interaction of surface water dynamics and the corresponding directly connected shallow groundwater aquifer. Runoff generation processes, as well as the extent and spatial distribution of the interaction between surface water and groundwater, are controlled by floodplain topography and by surface water dynamics. A modelling approach based on extensive experimental analyses is presented that describes the specific water balance of lowland areas, including the interactions of groundwater and surface water, as well as reflecting the important role of time‐variable shallow groundwater stages for runoff generation in floodplains. In the first part, experimental investigations of floodplain hydrological characteristics lead to a qualitative understanding of the water balance processes and to the development of a conceptual model of the water balance and groundwater dynamics of the study area. Thereby model requirements which allow for an adequate simulation of the floodplain hydrology, considering also interactions between groundwater and surface water have been characterized. Based on these analyses, the Integrated Modelling of Water Balance and Nutrient Dynamics (IWAN) approach has been developed. This consists of coupling the surface runoff generation and soil water routines of the deterministic, spatially distributed hydrological model WASIM‐ETH‐I with the three‐dimensional finite‐difference‐based numerical groundwater model MODFLOW and Processing MODFLOW. The model was applied successfully to a mesoscale subcatchment of the Havel River in northeast Germany. It was calibrated for two small catchments (1·4 and 25 km²), where the importance of the interaction processes between groundwater and surface waters and the sensitivity of several controlling parameters could be quantified. Validation results are satisfying for different years for the entire 198 km² catchment. The model approach was further successfully tested for specific events. The experimental area is a typical example of a floodplain‐dominated landscape. It was demonstrated that the lateral flow processes and the interactions between groundwater and surface water have a major importance for the water balance and periodically superimposed on the vertical runoff generation. Copyright © 2006 John Wiley & Sons, Ltd.     

Use of the MODFLOW 6 Water Mover Package to Represent Natural and Managed Hydrologic Connections

The latest release of MODFLOW 6, the current core version of the MODFLOW groundwater modeling software, debuted a new package dubbed the “mover” (MVR). Using a generalized approach, MVR facilitates the transfer of water among any arbitrary combination of simulated features (i.e., pumping wells, stream, drains, lakes, etc.) within a MODFLOW 6 simulation. Four “rules” controlling the amount of water transferred from a providing feature to a receiving feature are currently available. In this way, MVR can represent natural connections between features, for example, streams entering or exiting lakes, and perhaps more interestingly, it also can transfer water among simulated features to more accurately simulate water management. An example model representative of an agricultural setting demonstrates some of the available MVR connections. For example, an irrigation event that transfers surface water from an irrigation delivery ditch to multiple cropped areas demonstrates a “one-to-many” connection that is possible within MVR. Conversely, irrigation or precipitation runoff from multiple fields may be routed to a particular stream segment using “many-to-one” MVR connections. MVR supports many additional connection types, several of which are demonstrated by the included example problem.     

Land cover effects on runoff patterns in eastern Piedmont (USA) watersheds

Physiography and land cover determine the hydrologic response of watersheds to climatic events. However, vast differences in climate regimes and variation of landscape attributes among watersheds (including size) have prevented the establishment of general relationships between land cover and runoff patterns across broad scales. This paper addresses these difficulties by using power spectral analysis to characterize area‐normalized runoff patterns and then compare these patterns with landscape features among watersheds within the same physiographic region. We assembled long‐term precipitation and runoff data for 87 watersheds (first to seventh order) within the eastern Piedmont (USA) that contained a wide variety of land cover types, collected environmental data for each watershed, and compared the datasets using a variety of statistical measures. The effect of land cover on runoff patterns was confirmed. Urban‐dominated watersheds were flashier and had less hydrologic memory compared with forest‐dominated watersheds, whereas watersheds with high wetland coverage had greater hydrologic memory. We also detected a 10–15% urban threshold above which urban coverage became the dominant control on runoff patterns. When spectral properties of runoff were compared across stream orders, a threshold after the third order was detected at which watershed processes became dominant over precipitation regime in determining runoff patterns. Finally, we present a matrix that characterizes the hydrologic signatures of rivers based on precipitation versus landscape effects and low‐frequency versus high‐frequency events. The concepts and methods presented can be generally applied to all river systems to characterize multiscale patterns of watershed runoff. Copyright © 2013 John Wiley & Sons, Ltd.     

Mapped Predictions of Manganese and Arsenic in an Alluvial Aquifer Using Boosted Regression Trees

Manganese (Mn) concentrations and the probability of arsenic (As) exceeding the drinking-water standard of 10 μg/L were predicted in the Mississippi River Valley alluvial aquifer (MRVA) using boosted regression trees (BRT). BRT, a type of ensemble-tree machine-learning model, were created using predictor variables that affect Mn and As distribution in groundwater. These variables included iron (Fe) concentrations and specific conductance predicted from previously developed BRT models, groundwater flux and age estimates from MODFLOW, and hydrologic characteristics. The models also included results from the first airborne geophysical survey conducted in the United States to target an entire aquifer system. Predictions of high Mn and As occurred where Fe was high. Predicted high Mn concentrations were correlated with fraction of young groundwater (less than 65 years) computed from MODFLOW results. High probabilities of As exceedance were predicted where groundwater was relatively old and airborne electromagnetic resistivity was high, typically proximal to streams. Two-variable partial-dependence plots and sensitivity analysis were used to provide insight into the factors controlling Mn and As distribution in groundwater. The maps of predicted Mn concentrations and As exceedance probabilities can be used to identify areas where these constituents may be high, and that could be targeted for further study. This paper shows that incorporation of a selected set of process-informed data, such as MODFLOW results and airborne geophysics, into a machine-learning model improves model interpretability. Incorporation of process-rich information into machine-learning models will likely be useful for addressing a wide range of problems of interest to groundwater hydrologists.     

MODFLOW as a Configurable Multi-Model Hydrologic Simulator

MODFLOW 6 is the latest in a line of six “core” versions of MODFLOW released by the U.S. Geological Survey. The MODFLOW 6 architecture supports incorporation of additional hydrologic processes, in addition to groundwater flow, and allows interaction between processes. The architecture supports multiple model instances and multiple types of models within a single simulation, a flexible approach to formulating and solving the equations that represent hydrologic processes, and recent advances in interoperability, which allow MODFLOW to be accessed and controlled by external programs. The present version of MODFLOW 6 consolidates popular capabilities available in MODFLOW variants, such as the unstructured grid support in MODFLOW-USG, the Newton-Raphson formulation in MODFLOW-NWT, and the support for partitioned stress boundaries in MODFLOW-CDSS. The flexible multi-model capability allows users to configure MODFLOW 6 simulations to represent the local-grid refinement (LGR) capabilities available in MODFLOW-LGR, the multi-species transport capabilities in MT3DMS, and the coupled variable-density capabilities available in SEAWAT. This paper provides a new, holistic and integrated overview of simulation capabilities made possible by the MODFLOW 6 architecture, and describes how ongoing and future development can take advantage of the program architecture to integrate new capabilities in a way that is minimally invasive and automatically compatible with the existing MODFLOW 6 code.     

Uncertainty in hydrologic modelling for estimating hydrologic response due to climate change (Santiam River,Oregon)

This paper explores the predicted hydrologic responses associated with the compounded error of cascading global circulation model (GCM) uncertainty through hydrologic model uncertainty due to climate change. A coupled groundwater and surface water flow model (GSFLOW) was used within the differential evolution adaptive metropolis (DREAM) uncertainty approach and combined with eight GCMs to investigate uncertainties in hydrologic predictions for three subbasins of varying hydrogeology within the Santiam River basin in Oregon, USA. Predictions of future hydrology in the Santiam River include increases in runoff in the fall and winter months and decreases in runoff for the spring and summer months. One‐year peak flows were predicted to increase whereas 100‐year peak flows were predicted to slightly decrease. The predicted 10‐year 7‐day low flow decreased in two subbasins with little groundwater influences but increased in another subbasin with substantial groundwater influences. Uncertainty in GCMs represented the majority of uncertainty in the analysis, accounting for an average deviation from the median of 66%. The uncertainty associated with use of GSFLOW produced only an 8% increase in the overall uncertainty of predicted responses compared to GCM uncertainty. This analysis demonstrates the value and limitations of cascading uncertainty from GCM use through uncertainty in the hydrologic model, offers insight into the interpretation and use of uncertainty estimates in water resources analysis, and illustrates the need for a fully nonstationary approach with respect to calibrating hydrologic models and transferring parameters across basins and time for climate change analyses. Copyright © 2012 John Wiley & Sons, Ltd.     

Multivariate hydrologic time series analysis

The objective of this study is to establish a multivariate watershed hydrologic system model involving meteorological data as the input and river flow as the output of the system. Monthly hydrological time series of runoff, temperature and precipitation were selected for analysis. A first-order autoregressive-moving average (ARMA) transfer function model was found adequate to describe the multivariate watershed hydrologic system for the monthly runoff and meteorological time series. The results also indicated that the coordinated use of the meteorological and hydrometric data would enhance the accuracy of estimation of runoff characteristics.     

Assess hydrological responses to a warming climate at the Lysina Critical Zone Observatory in Central Europe

Climate warming is having profound effects on the hydrological cycle by increasing atmospheric demand, changing water availability, and snow seasonality. Europe suffered three distinct heat waves in 2019, and 11 of the 12 hottest years ever recorded took place in the past two decades, which will potentially change seasonal streamflow patterns and long-term trends. Central Europe exhibited six dry years in a row since 2014. This study uses data from a well-documented headwater catchment in Central Europe (Lysina) to explore hydrological responses to a warming climate. We applied a lumped parameter hydrologic model Brook90 and a distributed model Penn State Integrated Hydrologic Model (PIHM) to simulate long-term hydrological change under future climate scenarios. Both models performed well on historic streamflow and in agreement with each other according to the catchment water budget. In addition, PIHM was able to simulate lateral groundwater redistribution within the catchment validated by the groundwater table dynamics. The long-term trends in runoff and low flow were captured by PIHM only. We applied different EURO-CORDEX models with two emission scenarios (Representative Concentration Pathways RCP 4.5, 8.5) and found significant impacts on runoff and evapotranspiration (ET) for the period of 2071–2100. Results from both models suggested reduced runoff and increased ET, while the monthly distribution of runoff was different. We used this catchment study to understand the importance of subsurface processes in projection of hydrologic response to a warming climate.     

Does hillslope trenching enhance groundwater recharge and baseflow in the Peruvian Andes?

As Andean glaciers rapidly retreat due to climate change, the balance of groundwater and glacial meltwater contributions to stream discharge in tropical, proglacial watersheds will change, potentially increasing vulnerability of water resources. The Shullcas River Watershed, near Huancayo, Peru, is fed only partly by the rapidly receding Huaytapallana glaciers (<20% of dry season flow). To potentially increase recharge and therefore increase groundwater derived baseflow, the government and not‐for‐profit organizations have installed trenches along large swaths of hillslope in the Shullcas Watershed. Our study focuses on a nonglacierized subcatchment of the Shullcas River Watershed and has 2 objectives: (a) create a model of the Shullcas groundwater system and assess the controls on stream discharge and (b) investigate the impact of the infiltration trenches on recharge and baseflow. We first collected hydrologic data from the field including a year‐long hydrograph (2015–2016), meteorological data (2011–2016), and infiltration measurements. We use a recharge model to evaluate the impact of trenched hillslopes on infiltration and runoff processes. Finally, we use a 3‐dimensional groundwater model, calibrated to the measured dry season baseflow, to determine the impact of trenching on the catchment. Simulations show that trenched hillslopes receive approximately 3.5% more recharge, relative to precipitation, compared with unaltered hillslopes. The groundwater model indicates that because the groundwater flow system is fast and shallow, incorporating trenched hillslopes (~2% of study subcatchment area) only slightly increases baseflow in the dry season. Furthermore, the location of trenching is an important consideration: Trenching higher in the catchment (further from the river) and in flatter terrain provides more baseflow during the dry season. The results of this study may have important implications for Andean landscape management and water resources.     

Integrated Hydrologic Modeling to Untangle the Impacts of Water Management During Drought

Over the past century, groundwater levels in California's San Joaquin Valley have dropped by more than 30 m in some areas mostly due to excessive groundwater extraction used to irrigate agricultural lands and sustain a growing population. Between 2012 and 2015, California experienced the worst drought in its recorded history, depleting surface water supplies and further exacerbating groundwater depletion in the region. Due to a lack of groundwater regulation, exact quantities of extracted groundwater in California are unknown and hard to quantify. Recent adoption of the Sustainable Groundwater Management Act has intensified efforts to identify sustainable groundwater use. However, understanding sustainable use in a highly productive agricultural system with an extremely complex surface water allocation system, variable groundwater use, and spatially extensive and diverse irrigation practices is no easy task. Using an integrated hydrologic model coupled with a land surface model, we evaluated how water management activities, specifically a suite of irrigation and groundwater pumping scenarios, impact surface water–groundwater fluxes and storage components and how those activities and the relationships between them change during drought. Results showed that groundwater pumping volume had the most significant impact on long-term water storage changes. A comparison with total water storage anomaly (TWSA) estimates from NASA's Gravity Recover and Climate Experiment (GRACE) provided some insight regarding which combinations of pumping and irrigation matched the GRACE TWSA estimates, lending credibility to these scenarios. In addition, the majority of long-term water storage changes during the recent drought occurred in groundwater storage in the deeper subsurface.     

Hydrologic processes at the urban residential scale

In the face of increasing urbanization, there is growing interest in application of microscale hydrologic solutions to minimize storm runoff and conserve water at the source. In this study, a physically based numerical model was developed to understand hydrologic processes better at the urban residential scale and the interaction of these processes among different best management practices (BMPs). This model simulates hydrologic processes using an hourly interval for over a full year or for specific storm events. The model was applied to treatment and control single‐family residential parcels in Los Angeles, California. Data collected from the control and treatment sites over 2 years were used to calibrate and validate the model. Annual storm runoff to the street was eliminated by 97% with installation of rain gutters, a driveway interceptor, and lawn retention basin. Evaluated individually, the driveway interceptor was the most effective BMP for storm runoff reduction (65%), followed by the rain gutter installation (28%), and lawn converted to retention basin (12%). An 11 m³ cistern did not substantially reduce runoff, but provided 9% of annual landscape irrigation demand. Simulated landscape irrigation water use was reduced 53% by increasing irrigation system efficiency, and adjusting application rates monthly based on plant water demand. The model showed that infiltration and surface runoff processes were particularly sensitive to the soil's physical properties and its effective depth. Replacing the existing loam soil with clay soil increased annual runoff discharge to the street by 63% when climate and landscape features remained unchanged. Copyright © 2006 John Wiley & Sons, Ltd.     

An integral-balance nonlinear model to simulate changes in soil moisture,groundwater and surface runoff dynamics at the hillslope scale

We present a system of ordinary differential equations (ODEs) capable of reproducing simultaneously the aggregated behavior of changes in water storage in the hillslope surface, the unsaturated and the saturated soil layers and the channel that drains the hillslope. The system of equations can be viewed as a two-state integral-balance model for soil moisture and groundwater dynamics. Development of the model was motivated by the need for landscape representation through hillslopes and channels organized following stream drainage network topology. Such a representation, with the basic discretization unit of a hillslope, allows ODEs-based simulation of the water transport in a basin. This, in turn, admits the use of highly efficient numerical solvers that enable space–time scaling studies. The goal of this paper is to investigate whether a nonlinear ODE system can effectively replicate observations of water storage in the unsaturated and saturated layers of the soil. Our first finding is that a previously proposed ODE hillslope model, based on readily available data, is capable of reproducing streamflow fluctuations but fails to reproduce the interactions between the surface and subsurface components at the hillslope scale. However, the more complex ODE model that we present in this paper achieves this goal. In our model, fluxes in the soil are described using a Taylor expansion of the underlying storage flux relationship. We tested the model using data collected in the Shale Hills watershed, a 7.9-ha forested site in central Pennsylvania, during an artificial drainage experiment in August 1974 where soil moisture in the unsaturated zone, groundwater dynamics and surface runoff were monitored. The ODE model can be used as an alternative to spatially explicit hillslope models, based on systems of partial differential equations, which require more computational power to resolve fluxes at the hillslope scale. Therefore, it is appropriate to be coupled to runoff routing models to investigate the effect of runoff and its uncertainty propagation across scales. However, this improved performance comes at the expense of introducing two additional parameters that have no obvious physical interpretation. We discuss the implications of this for hydrologic studies across scales.     

Annual distribution of river runoff with estimated contribution of winter low-water season

Proportions of the overland, delayed, and groundwater runoff are given for different phases of the hydrological regimes of rivers under different landscape–climate conditions in Russia. The concept of runoff hydrological regime is in agreement with the formation features of the annual and seasonal runoff on small watersheds under current climate conditions. The increase in winter runoff is due to the impulse discharge regime of soil water and groundwater, which receive additional recharge during frequent thaws and at weak freezing of the aeration zone. Variations in the values of runoff, total precipitation, and mean temperature were shown to be both synchronous and periodic.