Watershed Response Research Articles

A season-specific, multi-site calibration strategy to study the hydrological cycle and the impact of extreme-flow events along an urban-to-agricultural gradient

Extreme-flow events have been a central point of interest in hydrology and recent developments suggest that their role could be critical in the overall functioning of watershed systems. The development of modelling techniques that can effectively reproduce the hydrological dynamics of watersheds during extreme events is one of the challenges of the contemporary modelling practice. In this study, we present a season-specific, multi-site calibration framework that accommodates the variability in the hydrological responses induced by the agricultural landscape changes during different periods of the year. Our case study is the Hamilton Harbour watershed in southern Ontario, Canada, where discharge data from three main tributaries and six gauging stations were available to constrain the Soil and Water Assessment Tool (SWAT) model. Two distinct values are assigned to the Curve Number for antecedent runoff condition II (CN2), relating surface runoff to precipitation as a function of land uses and soil characteristics, in order to reproduce flow patterns during the growing and dormant seasons. Overall, notwithstanding the model performance decline in the most urbanized catchment of Redhill Creek, our season-specific calibration strategy improved the predictive capacity of the model in the predominantly agricultural catchments of Grindstone Creek and Spencer Creek. Counter to our season-specific process characterization, an alternative calibration strategy postulating a differential watershed response, depending on the magnitude of individual precipitation events, appears to fair better in urbanized settings. The water budget displays significant disparities among the different land uses in the Hamilton Harbour watershed. Our analysis also suggests that the runoff generated from urban sites during the growing season (May–October), i.e., the period when the receiving waterbody is most sensitive to eutrophication, is six times higher than the average contribution from pasture, forest, and cropland areas. The potential implications of the on-going land-use and climate-change trends for the water cycle at both local and catchment scales are also discussed.

Identifying Key Hydrological Processes in Highly Urbanized Watersheds for Flood Forecasting with a Distributed Hydrological Model

The world has experienced large-scale urbanization in the past century, and this trend is ongoing. Urbanization not only causes land use/cover (LUC) changes but also changes the flood responses of watersheds. Lumped conceptual hydrological models cannot be effectively used for flood forecasting in watersheds that lack long time series of hydrological data to calibrate model parameters. Thus, physically based distributed hydrological models are used instead in these areas, but considerable uncertainty is associated with model parameter derivation. To reduce model parameter uncertainty in physically based distributed hydrological models for flood forecasting in highly urbanized watersheds, a procedure is proposed to control parameter uncertainty. The core concept of this procedure is to identify the key hydrological and flood processes in the highly urbanized watersheds and the sensitive model parameters related to these processes. Then, the sensitive model parameters are adjusted based on local runoff coefficients to reduce the parameter uncertainty. This procedure includes these steps: collecting the latest LUC information or estimating this information using satellite remote sensing images, analyzing LUC spatial patterns and identifying dominant LUC types and their spatial structures, choosing and establishing a distributed hydrological model as the forecasting tool, and determining the initial model parameters and identifying the key hydrological processes and sensitive model parameters based on a parameter sensitivity analysis. A highly urbanized watershed called Shahe Creek in the Pearl River Delta area was selected as a case study. This study finds that the runoff production processes associated with both the ferric luvisol and acric ferralsol soil types and the runoff routing process on urban land are key hydrological processes. Additionally, the soil water content under saturated conditions, the soil water content under field conditions and the roughness of urban land are sensitive parameters.

Hydrologic Response in an Urban Watershed as Affected by Climate and Land-Use Change

The change in both streamflow and baseflow in urban catchments has received significant attention in recent decades as a result of their drastic variability. In this research, effects of climate variation and dynamics of land use are measured separately and in combination with streamflow and baseflow in the Little Eagle Creek (LEC) watershed (Indianapolis, Indiana). These effects are examined using land-use maps, statistical tests, and hydrological modeling. Transition matrix analysis was used to investigate the change in land use between 1992 and 2011. Temporal trends and changes in meteorological data were evaluated from 1980–2017 using the Mann–Kendall test. Changes in streamflow and baseflow were assessed using the Soil and Water Assessment Tool (SWAT) hydrological model using multiple scenarios that varied in land use and climate change. Evaluation of the model outputs showed streamflow and baseflow in LEC are well represented using SWAT. During 1992–2011, roughly 30% of the watershed experienced change, typically cultivated agricultural areas became urbanized. Baseflow is significantly affected by the observed urbanization; however, the combination of land and climate variability has a larger effect on the baseflow in LEC. Generally, the variability in the baseflow and streamflow appears to be heavily driven by the response to climate change in comparison to variability due to altered land use. The results reported herein expand the current understanding of variation in hydrological components, and provide useful information for management planning regarding water resources, as well as water and soil conservation in urban watersheds in Indiana and beyond.

The Effect of QPF on Real-Time Deterministic Hydrologic Forecast Uncertainty

Abstract The use of quantitative precipitation forecast (QPF) in hydrologic forecasting is commonplace, but QPF is subject to considerable error. When QPF is included as a model forcing in the hydrological forecast process, significant error propagates through the hydrologic predictions. Two questions arise: 1) are the resulting observed hydrologic forecast errors sufficiently large to suggest the use of zero QPF in the forecast process, and 2) if the use of nonzero QPF is indicated, how many periods (hours) of QPF (1, 6, 12, …, 72 h) should be used? Also, do forecast conditions exist under which the use of QPF should be different? This study presents results from two real-time hydrologic forecast experiments, focused on the NOAA/NWS Ohio River Forecast Center (OHRFC). The experiments rely on forecasts from subbasins at 39 forecast point locations, ranging in drainage area, geographic location within the Ohio River Valley, and watershed response time. Results from an experiment, spanning all flow ranges, for the 10 August 2007–31 August 2009 period, show that nonzero QPF produces smaller hydrologic forecast error than zero QPF. A second experiment, 23 January 2009–15 September 2010, suggests that QPF should be limited to 6–12-h duration for flood forecasts. Beyond 12 h, hydrologic forecast error increases substantially across all forecast ranges, but errors are much larger for flood forecasts. Increased durations of QPF produce smaller forecast error than shorter QPF durations only for nonflood forecasts. Experimental results are shown to be consistent with NWS April 2001–October 2016 forecast verification statistics for the OHRFC.

Assessing the long-term impact of conservation agriculture on wheat-based systems in Tunisia using APSIM simulations under a climate change context.

Several circulation models are forecasting climate changes in the Mediterranean region. Accordingly, it is expected that water scarcity in the region will be higher with drastic shifts of hydrological and erosive watershed responses. In Tunisia, wheat yields have been variable over the years and are lower than the potential yields. In response, the adoption of conservation agriculture (CA), introduced into Tunisia in 1999 to help adaptation to climate change, has resulted in a substantial reduction in agricultural productivity. CA areas increased from 52 ha in 1999 to 14,000 ha in 2015. Using a modelling approach, the present paper evaluates the potential of CA to adapt wheat-based-systems to climate change in Tunisia. The Agricultural Production Systems Simulator (APSIM) model was used to predict the effect of tillage (conventional tillage [CT] vs. zero-tillage [ZT] and soil residue retention [ZT-RR]) on wheat productivity and soil fertility. Two contrasting locations in Tunisia were studied; one semi-arid (Kef) and one sub-humid (Bizerte). Results showed that the sustainable production of durum wheat under climate change conditions in Tunisia is possible through the adoption of CA practices (ZT and ZT-RR) in both sub-humid and semi-arid areas. In fact, mulching (residue retention) is more effective than CT (under semi-arid and sub-humid conditions) in enhancing wheat yield (15%), water use efficiency (18% and 13%) and soil organic carbon accumulation (0.13 t ha-1 year-1 and 0.18 t ha-1 year-1). It is also more effective for soil resilience - preventing water erosion (1.7 t ha-1 year-1 and 4.6 t ha-1 year-1 of soil loss). The present study allowed identification of 260,000 ha as priority areas for CA adoption; this represent one-third of the total cereal area in Tunisia. Appropriate evaluation of the benefits of CA on sustainable agricultural intensification would provide more arguments for effectively supporting CA adoption in Tunisia.

Effect of rapid urbanization on Mediterranean karstic mountainous drainage basins

Transformation of permeable natural areas into impervious surfaces (e.g. roads, parking lots, and buildings) is altering the watershed response to precipitation, generating bigger runoff volumes with increased peak discharges. The runoff travels faster to the watershed outlet, accumulates, and might cause flooding. Urban areas are affected by flooding, with consequences that might result in severe economic damage and even hazard to life. The paper will estimate the effect of rapid urbanization on runoff and on groundwater recharge. The model includes the Shiloh catchment basin located in a karstic mountainous terrain in Israel for a time period of rapid development from 1978 to 2018. Evaluation of surface permeability was based on land-use classifications from Landsat datasets using the Q-GIS 3.4 platform. Multiyear runoff coefficient estimation was calculated based on annual runoff and rainfall series. The evaluation suggests natural permeable areas were constantly declining from about 400 km2 (nearby 100% of the Shiloh basin) in 1978 to about 100 km2 in 2018 and that nonpermeable areas increased to 95 km2 and semi-permeable areas to 250 km2. Despite annual precipitation decrease during the last two decades, the annual runoff coefficient has a pronounced tendency to increase. This research suggests that the anthropogenic impact on the Shiloh basin and, hence, similar watersheds encourages runoff accumulation, resulting in extreme flood events recurrence while significantly reducing the groundwater recharge.

Deforestation impact on discharge regime in the Doce River Basin

The construction of scenarios using hydrological models can evaluate the hydrological response in watersheds, due to changes in the soil use. In this context, this study analyzed the consequences of deforestation in the hydrological behavior of the Doce River Basin, which has a drainage area of approximately 86.715 km². The basin presents problems regarding water availability, floods, indiscriminate deforestation and inadequate soil management. The Model of Large Basins (MGB-IPH) was selected, using daily data from 1990 to 2014, 11 fluviometric, 81 rainfall and 12 meteorological stations, numerical model of the land, soil maps, and use and land cover. Hydrological modeling was performed in the following steps: calibration of parameters (1990 and 2005), validation (2006 to 2014) and simulation of deforestation scenarios (2000 to 2014). It was observed that the replacement of forests by pasture caused reductions in the average annual flows, indicating a decrease in average flows in deforestation scenarios. As for the behavior of floods, deforestation caused them to increase, while the annual minimum flows reduced with deforestation. The results demonstrate the worsening that the simulated scenarios can cause in the problems already found in the basin, such as floods and water shortages, to supply the uses for which the basin is intended.

Assessment of hydrology and nutrient losses in a changing climate in a subsurface-drained watershed

Studies assessing the impact of subsurface drains on hydrology and nutrient yield in a changing climate are limited, specifically for Western Lake Erie Basin. This study aimed to evaluate the impact of changing climate on hydro-climatology and nutrient loadings in agricultural subsurface-drained areas on a watershed in northeastern Indiana. The study was conducted using a hydrologic model - the Soil and Water Assessment Tool (SWAT) - under two different greenhouse gas emission scenarios (RCP 4.5 and RCP 8.5). Based on analysis, annual subsurface drain flow totals could increase by 70% with respect to the baseline by the end of the 21st century. Surface runoff could increase by 10 to 140% and changes are expected to be greater under RCP 8.5. Soluble phosphorus yield over the basin in a year via subsurface drains could decrease by 30 to 60% under either emission scenarios. Annual total soluble phosphorus yield (soluble phosphorus loading to stream) from subsurface drains and surface runoff could vary from 0.041 to 0.058 kg/ha under RCP 4.5 and 0.035 to 0.064 kg/ha under RCP 8.5 by the end of the 21st century while the values from the baseline model were 0.051 kg/ha. This was attributable to the fact that future climate could have a greater increase in surface runoff than subsurface drain flow based on analysis of the different climate scenarios. Outputs from individual climate model data rather than ensembles provided a band of influence of watershed responses, while outputs from different timelines provided details for evaluating management practice suitability with respect to anticipated differences in climate. Results provide valuable information for stakeholders and policy makers for planning management practices to protect water quality.

Impact of Land Use/Land Cover Change on Hydrology of the Catchment: The Case of Upper Ribb Catchment, Lake Tana Sub Basin, Ethiopia

Land use/land cover dynamics has been recognized as one of the factors responsible for altering the hydrologic response of watersheds. Various extent of water resources projects planning and implementation will require knowledge of these changes on catchment hydrology. This study assesses the impact of land use and land cover change effects on stream flow using SWAT model in Upper Ribb Catchment. The analysis of stream flow was done for three different years of 1973, 1995 and 2016 using SWAT model. During the study period of over 43 years (1973 – 2016) stream flow was increased to 6.143m 3 /s due to an increase of cultivated land by 29.947%. Model calibration and validation for stream flow were done at Upper Ribb gauging station. The performance of the model was also checked at this station. Both the monthly calibration and validation results showed good match between measured and simulated stream flow data with the coefficient of determination (R 2 ) of 0.85, Nash-Sutcliffe efficiency (NSE) of 0.824 for the calibration, and R 2 of 0.829 and NSE of 0.814 of the validation period. Keywords : Upper Ribb, Catchment, SWAT, stream flow. DOI : 10.7176/JEES/9-6-02 Publication date :June 30 th 2019

Impacts of Land Cover/Use Changes on Hydrological Processes in a Rapidly Urbanizing Mid-latitude Water Supply Catchment

This research aimed to evaluate the impact of land cover/use changes on watershed responses and hydrological processes by applying the Soil and Water Assessment Tool (SWAT) distributed hydrologic model to the Buyukcekmece Water Basin of Istanbul Metropolitan city. SWAT model was run for two different scenarios for the 40-year period between 1973 and 2012, after completing calibration procedures under gauge-data scarce conditions. For the first scenario, 1990 dated Land cover/land use (LCLU) map and meteorological data obtained between 1973 and 2012 were used. For the second scenario, 2006 dated LCLU map and same meteorological data were used to analyze the impact of changing landscape characteristics on hydrological processes. In the selected watershed, LCLU changes started towards the end of the 1980s and reached a significant status in 2006; therefore, 1990 and 2006 dated LCLU maps are important to model human impact period in the watershed. Afterwards, LCLU changes within sub-basin level were investigated to quantify the effects of different types of land changes on the major hydrological components such as actual evapotranspiration, percolation, soil water, base flow, surface runoff and runoff. Our analysis indicated that, under the same climatic conditions, changes in land cover/use, specifically urbanization, played a considerable role in hydrological dynamics with changes on actual transpiration, base flow, surface runoff, runoff, percolation and soil water mainly due to urban and agricultural area changes. Among the different hydrological components analyzed at watershed level, percolation, ET and base flow were found to be highly sensitive to LCLU changes, whereas soil water was found as the least sensitive to same LCLU changes.

A Race Against Time: Modeling Time Lags in Watershed Response

AbstractLand use change and agricultural intensification have increased food production but at the cost of polluting surface and groundwater. Best management practices implemented to improve water quality have met with limited success. Such lack of success is increasingly attributed to legacy nutrient stores in the subsurface that may act as sources after reduction of external inputs. However, current water‐quality models lack a framework to capture these legacy effects. Here we have modified the SWAT (Soil Water Assessment Tool) model to capture the effects of nitrogen (N) legacies on water quality under multiple land‐management scenarios. Our new SWAT‐LAG model includes (1) a modified carbon‐nitrogen cycling module to capture the dynamics of soil N accumulation, and (2) a groundwater travel time distribution module to capture a range of subsurface travel times. Using a 502‐km2 Iowa watershed as a case study, we found that between 1950 and 2016, 25% of the total watershed N surplus (N Deposition + Fertilizer + Manure + N Fixation − Crop N uptake) had accumulated within the root zone, 14% had accumulated in groundwater, while 27% was lost as riverine output, and 34% was denitrified. In future scenarios, a 100% reduction in fertilizer application led to a 79% reduction in stream N load, but the SWAT‐LAG results suggest that it would take 84 years to achieve this reduction, in contrast to the 2 years predicted in the original SWAT model. The framework proposed here constitutes a first step toward modifying a widely used modeling approach to assess the effects of legacy N on the time required to achieve water‐quality goals.

Modeling Runoff and Nitrogen Loads From a Watershed at Different Levels of Impervious Surface Coverage and Connectivity to Storm Water Control Measures

AbstractUrban development of watersheds increases runoff and nitrogen loads by adding urban impervious surfaces and increasing the hydrologic connectivity of these surfaces to streams. Storm water control measures (SCMs) are designed to disrupt this connectivity by retaining water in biologically active depressions where nitrogen retention, transformation, and removal occur. This work applies a mechanistic, spatially distributed, hydroecological model (RHESSys) to a suburban watershed in Charlotte, NC, with 15% total imperviousness (TI) and 33% watershed area mitigated by SCMs. We developed emergent relationships between watershed‐scale predictors (TI and connectivity to SCMs) and water and nitrogen response variables (storm water runoff ratios and nitrogen load by species). Results showed that annual runoff ratios were insensitive to increases in connectivity to SCMs (varying by ~1% of rainfall) because SCMs did not substantially increase evaporation but that runoff ratios increased by an average 0.2% per 1% increase in TI due to decreases in transpiration in the watershed. Generally, nitrate loads increased with TI but decreased as more surfaces were mitigated by SCMs. However, these nitrate reductions corresponded to increased export of dissolved organic nitrogen and ammonium. Together, these results indicate that SCMs act as both removers and transformers of nitrogen at the watershed scale. SCMs showed a net assimilation of nitrogen in warm months and net release in cool months, which offset the timing of nitrogen export relative to inputs. This work highlights that using a hydroecological, process‐based model reveals both the emergent relationships between watershed condition and response and the processes controlling those relationships.

Hydrograph peak-shaving using a graph-theoretic algorithm for placement of hydraulic control structures

The need to attenuate hydrograph peaks is central to the design of stormwater and flood control systems. However, few guidelines exist for siting hydraulic control structures such that system-scale benefits are maximized. This study presents a new graph-theoretic algorithm for stabilizing the hydrologic response of watersheds by placing controllers at strategic locations in the drainage network. This algorithm identifies subcatchments that dominate the peak of the hydrograph, and then finds the “cuts” in the drainage network that maximally isolate these subcatchments, thereby flattening the hydrologic response. Evaluating the performance of the algorithm through an ensemble of hydrodynamic simulations, we find that our controller placement algorithm produces consistently flatter discharges than randomized controller configurations—both in terms of the peak discharge and the overall variance of the hydrograph. By attenuating flashy flows, our algorithm provides a powerful methodology for mitigating flash floods, reducing erosion, and protecting aquatic ecosystems. More broadly, we show that controller placement exerts an important influence on the hydrologic response and demonstrate that analysis of drainage network structure can inform more effective stormwater control policies.

Simulating the influence of integrated crop-livestock systems on water yield at watershed scale

Integrated crop-livestock (ICL) systems are being promoted as environmentally favorable alternatives to traditional crop agriculture and livestock production. There are few, if any, evaluation studies of the hydrologic response of watersheds to the implementation of ICL systems. Thus, we applied the Soil and Water Assessment Tool (SWAT) model to simulate the potential impacts of ICL systems on water yield and its hydrological components using a large agricultural dominated watershed. In this study, the integration of grazing operations with cropping systems represented cattle grazing under three typical crop rotations: (i) continuous corn (Zea mays L.; 1-year rotation), (ii) conventional (corn-soybean [Glycine max (L.) Merr.]; 2-year rotation), and (iii) winter cover crops (corn-soybean-oats (Avena sativa L.)/winter barley (Hordeum vulgare L.); 3-year rotation). Modeling results showed a significant reduction in water yield over a long-term period simulation (31 years) when grazing of corn residue or winter barley was scheduled within the rotations. When compared to scenarios without grazing operations, the reduction in water yield was 14.7% under corn-soybean rotation (corn as the forage grazed), 12.5% under continuous corn rotation, 6.4% under corn-soybean-oats/winter barley rotation (corn as the forage grazed), and 3% under corn-soybean-oats/winter barley rotation (winter barley as the forage grazed). Of the three components that constitute water yield (i.e., surface runoff, lateral and groundwater flow), only surface runoff was reduced when integrating grazing into the cropping system. Instead, lateral and groundwater flows increased when ICL systems were scheduled in the watershed. Groundwater flow was the hydrological component with the highest relative impact on streamflow. These results indicate that ICL systems can positively affect processes involved in soil water storage and transit. Runoff reduction benefits of ICL systems might be helpful in improving the environmental quality of receiving waterbodies and in reducing flood-risk potential. These systems over the long-term could benefit the watershed's hydrological cycle through increased baseflow. Overall, this study suggests new watershed-scale benefits of ICL systems with important hydrological implications that might be of interest for agricultural watershed planners.

Long-Term Changes of Hydrological Variables in a Small Lowland Watershed in Central Poland

Climate-induced changes in small watersheds are still not well researched because long-term datasets are usually missing for these watersheds. Such studies can, however, improve our understanding of the watershed response to climatic changes at such a small scale being transparent. In this study, we investigate trends in temperature, precipitation and river-flow characteristics in a small watershed, typical for Central Poland, with 53 years of observations (1963–2015) using the Mann-Kendall test. Particularly, we examine whether any trends in hydro-meteorological variables can be identified, and if any associated changes in water resources in this region can already be observed. We found that this short period already allows for detecting some changes in hydro-meteorological variables. These changes could be characterized by a significant increase in the mean annual air temperature on a daily basis, and a significant decrease in the mean annual discharge on a daily basis and in the minimum annual discharge on a daily basis. Yet, no significant trend could be detected for the total annual precipitation, the maximum summer rainfall, and the maximum annual discharge on a daily basis. These findings indicate that water resources are decreasing in this region, which affects natural habitats, agriculture and local communities.

Enhancement of primary production during drought in a temperate watershed is greater in larger rivers than headwater streams

AbstractDrought is common in rivers, yet how this disturbance regulates metabolic activity across network scales is largely unknown. Drought often lowers gross primary production (GPP) and ecosystem respiration (ER) in small headwaters but by contrast can enhance GPP and cause algal blooms in downstream estuaries. We estimated ecosystem metabolism across a nested network of 13 reaches from headwaters to the main stem of the Connecticut River from 2015 through 2017, which encompassed a pronounced drought. During drought, GPP and ER increased, but with greater enhancement in larger rivers. Responses of GPP and ER were partially due to warmer temperatures associated with drought, particularly in the larger rivers where temperatures during summer drought were > 10°C higher than typical summer baseflow. The larger rivers also had low canopy cover, which allowed primary producers to take advantage of lower turbidity and fewer cloudy days during drought. We conclude that GPP is enhanced by higher temperature, lower turbidity, and longer water residence times that are all a function of low discharge, but ecosystem response in temperate watersheds to these drivers depends on light availability regulated by riparian canopy cover. In larger rivers, GPP increased more than ER during drought, even leading to temporary autotrophy, an otherwise rare event in the typically light‐limited heterotrophic Connecticut River main stem. With climate change, rivers and streams may become warmer and drought frequency and severity may increase. Such changes may increase autotrophy in rivers with broad implications for carbon cycling and water quality in aquatic ecosystems.

The Po River Water Isotopes during the Drought Condition of the Year 2017

The year 2017 was anomalously warm and dry across the whole Italian Peninsula, and the paucity of precipitation was emphasized during the extreme summer drought of the main Italian river, i.e., the Po river, which was characterized by a discharge persistently below 600 m3/s (in spite of the average discharge of 1500 m3/s). During these extreme conditions, the Po river oxygen and hydrogen stable isotopes (δD, δ18O) displayed a relation (δD = 6.1*δ18O − 6.6) significantly different from that recorded in the previous investigation of the years 2012–2014 (δD = 7.5*δ18O + 6.5). The lowering of the slope and the negative intercept clearly reflect the transition toward arid conditions that characterized the investigated period. The difference is expressed by the derivative parameter Line-Conditioned excess (LC-exc), which better describes the compositional difference of Po river water in the year 2017 with respect to that of the period 2012–2014, when the system was less affected by warm or dry conditions and the river discharge was more similar to the historical trends. The isotopic anomaly observed in 2017 throughout the river is even greater in the terminal part of the river, where in the meanders of the deltaic branches, the river flow progressively slows down, suffering significant evaporation. The isotopic signature of the water appears, therefore, an appropriate tool to monitor the watershed response to evolving environmental conditions. These sensitive isotopic parameters could be interpreted as “essential climate variables” (ECV) that are physical, chemical, or biological geo-referenced parameters that critically contribute to the characterization of Earth’s climate. Future research needs to find relationships between ECV (including the water stable isotopes) and the evolution of ecosystems, which especially in the Mediterranean area, appear to be fragile and severely affected by natural and anthropogenic processes.

On the linkage between runoff generation, land drainage, soil properties, and temporal patterns of precipitation in agricultural floodplains

Accurate awareness of how rainfall, land use changes, and soil types control water fluxes in agricultural floodplains remains a crucial challenge in water resource research. This study examines soil moisture conditions, soil texture and rainfall characteristics, together with different artificial drainage network structures covering a time-span of 100 years (1924–2010), as drivers for runoff production in an agricultural floodplain. The research incorporates a multiple-layer generalised Green-Ampt approach to simulate water infiltration into the ground. Once the storage offered by the soil is saturated, a portion of the surface storage provided by the drainage network satisfies the infiltration capacity, thus delaying runoff. The watershed response is defined by the uNSI (updated Network Saturated index, (Sofia and Tarolli, 2017), that indicates the moment the available storage (soil + network) is 100% saturated. The results highlighted how interlocking relations between soil properties, the geometry of the network and temporal variations of precipitation determine runoff generation timing. For short return times, intense rainfalls tend to produce a quicker response in areas with soils prone to saturation, and with decreased network complexity. However, when the event magnitude increases, this combination of soil and network structure produces the fastest response when rainfall is more regular. Intense events in zones with soils with higher permeability produce a quicker response the simpler the network is. When soils are prone to runoff, and the network efficiency increases, runoff production is delayed in time. When soils have elevated permeability, and the network has a reduced efficiency and path heterogeneity, increasing the network simplicity would result in similar outcomes. Moreover, if the path heterogeneity and network efficiency increases, for a given network sinuosity, runoff generation would be delayed. Quantifying these effects is indeed crucial for many environmental problems, including the prediction of impacts of a changing climate and land use and the associated pressures.

Evaluation and Benchmarking of Operational Short-Range Ensemble Mean and Median Streamflow Forecasts for the Ohio River Basin

Abstract This study presents findings from a real-time forecast experiment that compares legacy deterministic hydrologic stage forecasts to ensemble mean and median stage forecasts from the NOAA/NWS Meteorological Model-Based Ensemble Forecast System (MMEFS). The NOAA/NWS Ohio River Forecast Center (OHRFC) area of responsibility defines the experimental region. Real-time forecasts from subbasins at 54 forecast point locations, ranging in drainage area, geographic location within the Ohio River valley, and watershed response time serve as the basis for analyses. In the experiment, operational hydrologic forecasts, with a 24-h quantitative precipitation forecast (QPF) and forecast temperatures, are compared to MMEFS hydrologic ensemble mean and median forecasts, with model forcings from the NOAA/NWS National Centers for Environmental Prediction (NCEP) North American Ensemble Forecast System (NAEFS), over the period from 30 November 2010 through 24 May 2012. Experiments indicate that MMEFS ensemble mean and median forecasts exhibit lower errors beginning at about lead time 90 h when forecasts at all locations are aggregated. With fast response basins that peak at ≤24 h, ensemble mean and median forecasts exhibit lower errors much earlier, beginning at about lead time 36 h, which suggests the viability of using MMEFS ensemble forecasts as an alternative to OHRFC legacy forecasts. Analyses show that ensemble median forecasts generally exhibit smaller errors than ensemble mean forecasts for all stage ranges. Verification results suggest that OHRFC MMEFS NAEFS ensemble forecasts are reasonable, but needed improvements are identified.

Hydrological response of a large-scale mountainous watershed to rainstorm spatial patterns and reforestation in subtropical China

This paper aims to investigate the hydrological response of a large-scale (8973 km2) mountainous watershed to different rainstorm spatial patterns and reforestation. Based on 32 years of observations, measurements of 184 rainstorm events and 125 sediment-producing events with complete hydrographs were analyzed. The K-means clustering method was used to classify the spatial patterns of rainstorm events in accordance with their event-based spatial rainfall characteristics. The 184 rainstorm events were classified into four spatial patterns, among which the spatial features differ significantly: (I) Spatial Pattern I (SPI) includes rainstorms with a low amount of cumulative areal rainfall (27.4 mm), the highest spatial variability (0.986), and the highest frequency; (II) Spatial Pattern II (SPII) includes rainstorms of high spatial variability (0.759) and the largest amount of local maximum daily rainfall (106.8 mm); (III) Spatial Pattern III (SPIII) includes rainstorms with a medium amount of cumulative areal rainfall (58.7 mm) and low spatial variability (0.362); and (IV) Spatial Pattern IV (SPIV) includes rainstorms with the largest amount of cumulative areal rainfall (117.2 mm) and the lowest spatial variability (0.313). Vegetation cover in the upper Du watershed was significantly improved after the implementation of the Grain-for-Green project. The average area-specific sediment yields (SSY) for the four SPs were 15.4, 65.5, 55.8, and 286.2 t km−2 before reforestation and decreased to 6.0, 59.3, 43.7 and 89.9 t km−2, respectively, after reforestation. ANOVA (analysis of variance) indicated that reforestation resulted in a significant reduction in runoff coefficient under SPIII and SPIV and a significant reduction in SSY under SPI and SPIV. A hysteresis analysis suggested that the proportion of events with a clockwise loop increased from 64.9% before reforestation to 82.1% after reforestation and that complex loops became less common during 2000–2010 under SPIV, thereby implying a reduced sediment supply.