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Remote sensing precipitation or precipitation from numerical weather prediction (NWP) is considered to be the best substitute for in situ ground observations for flood simulations in transboundary, data-scarce catchments. This research was aimed to evaluate the possibility of using a combination of a satellite precipitation product and NWP precipitation for better flood forecasting in the transboundary Chenab River Basin (CRB) in Pakistan. The gauge-calibrated satellite precipitation product, i.e., Global Satellite Mapping of Precipitation (GSMaP_Gauge), was selected to calibrate the Integrated Flood Analysis System (IFAS) model for the 2016 flood event in the Chenab River at the Marala Barrage gauging site in Pakistan. Precipitation from the Global Forecast System (GFS) NWP, with nine different lead times up to 4 days, was used in the calibrated IFAS model to predict the flood hydrograph in the Chenab River. The hydrologic simulations, with global GFS forecasts, were unable to predict the flood peak for all lead times. Then, the Weather Research and Forecasting (WRF) model was used to downscale the precipitation forecasts with one-way and two-way nesting approaches. In the WRF model, the CRB was centered in two domains of 25 km and 5 km resolutions. The downscaled precipitation forecasts were subsequently supplied to the IFAS model, and the predicted simulations were compared to obtain the optimal flood peak simulation in the Chenab River. It was found in this study that the simulated hydrographs, at different lead times, from the precipitation of two-way WRF nesting exhibited superior performance with the highest R2 and Nash–Sutcliffe efficiency (NSE) and the lowest percent bias (PBIAS) compared with one-way nesting. Moreover, it was concluded that the combination of GFS forecast and two-way WRF nesting can provide high-quality precipitation prediction to simulate flood hydrographs with a remarkable lead time of 96 h when applying coupled hydrometeorological flow simulation. [ABSTRACT FROM AUTHOR]

This study proposes the assessment of SWAT model simulations, with the provision of satellite precipitation products (SPPs), in a transboundary/large catchment. Three latest sub-daily/half-hourly (HH) and daily (D) SPPs, i.e., ‘IMERG-E’, ‘IMERG-L’, and ‘IMERG-F’, were evaluated for daily and monthly flow simulations. The study revealed that monthly flow simulation performance is better than daily flow simulation in all sub-daily and daily SPPs-based models. Results depict that IMERG-HHF and IMERG-DF yield the best performance among the other latency levels of SPPs. The IMERG-HHF model has a reasonably higher daily correlation coefficient (R) and lower daily root-mean-square error (RMSE) than IMERG-DF. IMERG-HHF displays the lowest percent bias (PBIAS) values of 15.4 and 2.4 for daily and monthly flow validation, respectively. It also represents relatively higher values of coefficient of determination (R2) and Nash–Sutcliffe Efficiency (NSE) than any other model, i.e., R2=0.66 and NSE=0.63 for daily model validation and R2=0.84 and NSE=0.82 for monthly model validation. Moreover, the sub-daily IMERG model outperformed the daily IMERG model for all calibration and validation scenarios. The IMERG-DL model demonstrates poor performance in all of the SPPs, in daily and monthly validation, with low R2 (0.63 (dval) and 0.81 (mval)), low NSE (0.50 (dval) and 0.67 (mval)), and high PBIAS (31 (dval) and 26.6 (mval)). Additionally, the IMERG-HHE model outperformed IMERG-HHL. HIGHLIGHTS Improvement in the SWAT daily model to set up the sub-daily model for a relatively large transboundary river catchment.; Daily and sub-daily satellite rainfall input comparison in the SWAT model for flow simulation.;

Remote sensing precipitation or precipitation from numerical weather prediction (NWP) is considered to be the best substitute for in situ ground observations for flood simulations in transboundary, data-scarce catchments. This research was aimed to evaluate the possibility of using a combination of a satellite precipitation product and NWP precipitation for better flood forecasting in the transboundary Chenab River Basin (CRB) in Pakistan. The gauge-calibrated satellite precipitation product, i.e., Global Satellite Mapping of Precipitation (GSMaP_Gauge), was selected to calibrate the Integrated Flood Analysis System (IFAS) model for the 2016 flood event in the Chenab River at the Marala Barrage gauging site in Pakistan. Precipitation from the Global Forecast System (GFS) NWP, with nine different lead times up to 4 days, was used in the calibrated IFAS model to predict the flood hydrograph in the Chenab River. The hydrologic simulations, with global GFS forecasts, were unable to predict the flood peak for all lead times. Then, the Weather Research and Forecasting (WRF) model was used to downscale the precipitation forecasts with one-way and two-way nesting approaches. In the WRF model, the CRB was centered in two domains of 25 km and 5 km resolutions. The downscaled precipitation forecasts were subsequently supplied to the IFAS model, and the predicted simulations were compared to obtain the optimal flood peak simulation in the Chenab River. It was found in this study that the simulated hydrographs, at different lead times, from the precipitation of two-way WRF nesting exhibited superior performance with the highest R2 and Nash–Sutcliffe efficiency (NSE) and the lowest percent bias (PBIAS) compared with one-way nesting. Moreover, it was concluded that the combination of GFS forecast and two-way WRF nesting can provide high-quality precipitation prediction to simulate flood hydrographs with a remarkable lead time of 96 h when applying coupled hydrometeorological flow simulation.

In small catchments, the time interval most commonly used for simulation – daily or monthly – may not be sufficient to accurately capture the time distribution of hydrological processes. In this paper, the Soil and Water Assessment Tool (SWAT) was used to perform an hourly long-term streamflow and sediment load simulation in the small (4.8 km2) and forested Aixola catchment (northern Spain). From this simulation, 10 runoff events were tested; the most satisfactory results for streamflow were obtained under wet antecedent conditions. However, simulated sediment load was underestimated during the peaks and remained high towards the end of the event. Furthermore, the influence of the precipitation time step (1–4 h, daily) was not relevant in the streamflow simulation but does influence the sediment simulation. The best results were achieved with the daily step simulations obtained at an hourly time step. This paper shows that sub-daily modelling improves water and especially sediment yield results; however, improvements are still needed in timing-related routines. [ABSTRACT FROM AUTHOR]

The Soil and Water Assessment Tool (SWAT) is a watershed-scale hydrologic model that integrates water quantity and quality modules. Despite the large amount of knowledge on the SWAT model, specific understanding of sub-daily applications remains limited. In this review, we identify the shortcomings and possible ways forward in simulating sub-daily processes with the model. A literature review was conducted, along with a participatory method based on a questionnaire. We reviewed 28 scientific articles and categorized them into: (i) model development, (ii) streamflow methods comparison, (iii) water quality, and (iv) other applications. We found that using sub-daily data improves hydrograph peak simulation, while for medium flows use of daily data was better. From all the reviewed studies, a 1-hour time step was the most suitable time scale for the sub-daily model application. The participatory questionnaire confirmed the hypothesis that the main challenge for using the sub-daily routine was the lack of high-resolution data. [ABSTRACT FROM AUTHOR]

Abstract Flash floods cause severe natural disasters over the world generating property and infrastructures damages, poverty and loss of human life, among others. Mediterranean coastal watersheds are specially sensible to flash floods effects due to their typical drainage basin features (steep slopes, short concentration times, complex orography, etc.) and the high rainfall intensity typical of convective systems. In the present study, the temporal evolution of the hydrological answer in five Mediterranean (SE Spain) ravine basins with sizes from 10.2 km2 to 200.9 km2 were analysed. A sub-daily SWAT model was used at watershed scale to capture the complex hydrological dynamics. Five land use scenarios corresponding to no-urbanization (baseline), 1990, 2000, 2006 and 2012 were evaluated using GIS-based tools. Additionally, statistical significant differences among the studied scenarios were checked employing the Kruskal-Wallis and post-hoc tests based on Mann-Whitney test with BH correction. Our results show that flash flood risks have increased in the studied catchments due to changes in land uses, particularly affected by a large urban growth. Observed changes in soil uses have been important since the sixties of the last century, because of a high demographic and touristic pressure and specially the urban area has enhanced considerably during the last 22 years. Currently, some of these catchments present around 70% of their surface occupied by urban land uses with high population density producing curve number surpasses 85 and 90 levels. The hydrological response of the studied basins changed to higher flow rate peaks and shorter concentration times. Some discharges increased significantly from the baseline land use scenario (≈50 m3/s, 190 m3/s, 380 m3/s) to the most urbanised scenario (≈235 m3/s, 385 m3/s, 940 m3/s), respectively. These findings provide to urban planning policy makers very useful information in the face of flash flood effects, which have cost even human lifes in the studied ravine basins during last years. Graphical abstract Unlabelled Image Highlights • High urban development in the coastline of SE Spain increases curve number rates (80–90). • In Mediterranean convective systems, some precipitation episodes surpass 200–300 mm in 3 or 4 h. • Flash floods simulations, through the SWAT code, reported discharge rates which exceed 900 m3/s. • Peak flow rates have doubled or tripled as a consequence of changes in the land use during last decades. • Improving hydraulic barriers and socio-environmental risk perception against flash floods effects is mandatory. [ABSTRACT FROM AUTHOR]

The current FORTRAN code of SWAT (SWAT2012) is being rebuilt to ease new model and module development by incorporating object-oriented programming techniques into defining spatial objects and calculating variables. SWAT+, the new SWAT code, is expected to offer flexible watershed representation with linked spatial objects. Ideally, SWAT+ produces outputs that are comparable to SWAT2012, if not identical, when the two versions of SWAT are set with the same input for a watershed. However, the drastic restructuring of the source code may cause unexpected results in the output, especially when compared with the current SWAT2012 model that has a long history of model validation. In this technical note, we compare sub-daily, daily, and monthly hydrographs provided by SWAT2012 and SWAT+ that were prepared for a small urban watershed in Austin, Texas. The model parameters were calibrated to 15 min and daily runoff observed in the watershed. SWAT+ was on par with SWAT2012 in predicting streamflow at daily and monthly scales. However, the HRU-level flow routing scheme newly added to SWAT+ created multiple peaks in the sub-daily hydrographs, suggesting a need to further improve the water transport simulation strategies. This technical note provides ideas of how SWAT+ is organized and if a SWAT modeler can maintain consistency in modeling results when shifting from the original code to the new code. In addition, this technical note discusses the potential for improved SWAT+ modeling. [ABSTRACT FROM AUTHOR]

Flash floods are natural phenomena with environmental, social and economic impacts. To date, few numerical models are able to simulate hydrological processes at catchment scale at a reasonable time scale to describe flash events with accurate details. Considering a ~810 km² Mediterranean river coastal basin (southwestern France) as a study case, the objective of the present study was to assess the ability of the sub-daily module of the lumped Soil andWater Assessment Tool (SWAT) to simulate discharge (1) time-continuously, by testing two sub-basin delineation schemes, two catchment sizes, and two output time-steps; and (2) at flood time-scale, by comparing the performances of SWAT to the performances of the event-based fully distributed MARINE model when simulating flash flood events. We showed that there was no benefit of decreasing the size of the minimum drainage area (e.g., from ~15 km² down to ~1 km²) when delineating sub-basins in SWAT. We also showed that both the MARINE and SWAT models were equally able to reproduce peak discharge, flood timing and volume, and that they were both limited by rainfall and soil data. Hence, the SWAT model appears to be a reliable modelling tool to predict discharge over long periods of time in large flash-flood-prone basins. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate the influence of sub-daily precipitation time steps on model performance and hydrological components by applying the Green and Ampt infiltration method using the Soil and Water Assessment Tool (SWAT). Precipitation was measured at a resolution of 0.1 mm and aggregated to 5-, 15-, 30-, and 60-min time steps. Daily discharge data over a 10-year period were used to calibrate and validate the model. Following a global sensitivity analysis, relevant parameters were optimized through an automatic calibration procedure using SWAT- CUP for each time step. Daily performance statistics were almost equal among all four time steps ( NSE ≈ 0.47). Discharge mainly consisted of groundwater flow (55%) and tile flow (42%), in reasonable proportions for the investigated catchment. In conclusion, model outputs were almost identical, showing simulations responded nearly independently of the chosen precipitation time step. This held true for (1) the selection of sensitive parameters, (2) performance statistics, (3) the shape of the hydrographs, and (4) flow components. However, a scenario analysis revealed that the precipitation time step becomes important when saturated hydraulic conductivities are low and curve numbers are high. The study suggests that there is no need in using precipitation time steps <1 h for lowland catchments dominated by soils with a low surface runoff potential if daily flow values are being considered. Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series. [ABSTRACT FROM AUTHOR]

A study was undertaken for the prediction of runoff flow from 0.8 ha field-sized agricultural watershed in South Korea using Soil and Water Assessment Tool (SWAT) sub-daily. The SWAT model with sub-daily configuration predicted flow from the watershed within the range of acceptable accuracy. The SWAT sub-daily simulations were carried out for a total of 18 rainfall events, 9 each for calibration and validation. Overall trend and extent of matching simulated flow for the rainfall events in 2007-2008 with measured data during the calibration process were coefficient of determination ( R) value of 0.88 and Nash and Sutcliffe Efficiency ( E) value of 0.88. For validation, R and E values were 0.9 and 0.84, respectively. Whereas R and E values for simulation results using daily rainfall data were 0.79 and −0.01, respectively, that were observed to be out of acceptable limits for the model simulation. The importance of higher time resolution (hourly) precipitation records for flow simulation were evaluated by comparing R and E with 15 min, 2 h, 6 h and 12 h precipitation data, which resulted in lower statistics with increases in time resolution of precipitation data. The SWAT sub-daily sensitivity analysis was performed with the consideration of hydraulic parameter and was found as in the rank order of CN2 (curve number), ESCO (soil evaporation compensation factor), GW_DELAY (ground water delay time), ALPHA_BF (base flow alpha factor), GWQMN (a threshold minimum depth of water in the shallow aquifer required for return flow to occur), REVAPMN (minimum depth of water in shallow aquifer for re-evaporation to occur), LAT_TIME (lateral flow travel time) respectively. These sensitive parameters were evaluated at 10% higher and lower values of the parameters, corresponding to 70.5% higher and 23.2% lower in simulated flow out from the SWAT model. From the results obtained in this study, hourly precipitation record for SWAT sub-daily with Green-Ampt infiltration method was proven to be efficient for runoff estimation at field sized watershed with higher accuracies that could be efficiently used to develop sitespecific Best Management Practices (BMPs) considering rainfall intensity, rather than simply using daily rainfall data. [ABSTRACT FROM AUTHOR]