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[Excerpt] Asia is one of the world's most flood-prone regions by many metrics: high flood magnitudes, frequency, severity; the number countries affected, the area of inundation; the number of people at risk; and importantly, flood-related fatalities (AIR, 2014; Luo, Maddoks, Iceland,Ward, & Winsemius, 2015; Table 1). With respect to mortality, nearly all the countries with more than 5,000 flood-related deaths since 1985 are from Asia (11 of 13; Table 1; Figure 1). As we write this commentary, flooding associated with tropical storm Nangka has caused more than 40 deaths in Laos, Cambodia, and Vietnam(Floodlist, 2020).

This preliminary investigation of the recent spate of deadly flash floods and debris flows in Ladakh (India) over the last decade identifies uncontrolled development in hazardous locations as an important factor contributing to loss of life and property damage in this high mountain desert. The sediments exposed in the channel banks and on the alluvial fans of several mountain streams in the area indicate a long history of flash floods and debris flows resulting from intense storms, which appear to have increased in frequency within the last decade. The signposts of these recurrent hazards are being ignored as a growing economy, which is boosted by a well-established tourism industry, is now driving development onto lands that are susceptible to floods and debris flow hazards. In this science briefing we argue that the increasing vulnerability in Ladakh should be addressed with sound disaster governance strategies that are proactive, rather than reactionary. Copyright © 2016 John Wiley & Sons, Ltd.

Alan D. Ziegler,* Robert J. Wasson, Alok Bhardwaj, Y. P. Sundriyal, S. P. Sati, Navin Juyal, Vinod Nautiyal, Pradeep Srivastava, Jamie Gillen and Udisha Saklani 1 National University of Singapore, Geography Department, Singapore 2 Institute of Water Policy in the Lee Kuan Yew School of Public Policy, National University of Singapore, Singapore 3 Department of Geology, HNB Garhwal University, Srinagar Garhwal, Uttarakhand, India 4 Physical Research Laboratory, Navrangpura, Ahmedabad, India 5 Department of Ancient History and Archaeology, HNB Garhwal University, Srinagar-Garhwal, Uttarakhand, India 6 Wadia Institute of Himalayan Geology, Dehra Dun, Uttarakand, India

Highland agriculture is intensifying rapidly in South-East Asia, leading to alarmingly high applications of agrochemicals. Understanding the fate of these contaminants requires carefully planned monitoring programmes and, in most cases, accurate simulation of hydrological pathways into and through water bodies. We simulate run-off in a steep mountainous catchment in tropical South-East Asia. To overcome calibration difficulties related to the mountainous topography, we introduce a new calibration method, named A Nash–Sutcliffe Efficiency Likelihood Match (ANSELM), that allows the assignment of optimal parameters to different hydrological response units in simulations of stream discharge with the Soil and Water Assessment Tool (SWAT) hydrological model. ANSELM performed better than the Parasol calibration tool built into SWAT in terms of model efficiency and computation time. In our simulation, the most sensitive model parameters were those related to base flow generation, surface run-off generation, flow routing and soil moisture change. The coupling of SWAT with ANSELM yielded reasonable simulations of both wet-season and dry-season storm hydrographs. Nash–Sutcliffe model efficiencies for daily stream flow during two validation years were 0.77 and 0.87. These values are in the upper range or even higher than those reported for other SWAT model applications in temperate or tropical regions. The different flow components were realistically simulated by SWAT, and showed a similar behaviour in all the study years, despite inter-annual climatic differences. The realistic partitioning of total stream flow into its contributing components will be an important factor for using this hydrological model to simulate solute transport in the future. Copyright © 2014 John Wiley & Sons, Ltd.

To investigate the effects of anthropogenic activity, namely, land use change and reservoir construction, on particulate organic carbon (POC) transport, we collected monthly water samples during September 2007 to August 2009 from the Longchuanjiang River to understand seasonal variations in the concentrations of organic carbon species and their sources and the yield of organic and inorganic carbon from the catchment in the Upper Yangtze basin. The contents of riverine POC, total organic carbon and total suspended sediment (TSS) changed synchronously with water discharge, whereas the contents of dissolved organic carbon had a small variation. The POC concentration in the suspended sediment decreased non-linearly with increasing TSS concentration. Higher molar C/N ratio of particulate organic matter (average 77) revealed that POC was dominated by terrestrially derived organic matter in the high flows and urban wastewaters in the low flows. The TSS transported by this river was 2.7 × 105 t/yr in 2008. The specific fluxes of total organic carbon and dissolved inorganic carbon (DIC) were 5.6 and 6 t/km2/yr, respectively, with more than 90% in the high flow period. A high carbon yield in the catchment of the upper Yangtze was due to human-induced land use alterations and urban wastes. Consistent with most rivers in the monsoon climate regions, the dissolved organic carbon–POC ratio of the export flux was low (0.41). Twenty-two percent (0.9 t/km2/yr) of POC out of 4 t/km2/yr was from autochthonous production and 78% (3.1 t/km2/yr) from allochthonous production. The annual sediment load and hence the organic carbon flux have been affected by environmental alterations of physical, chemical and hydrological conditions in the past 50 years, demonstrating the impacts of human disturbances on the global and local carbon cycling. Finally, we addressed that organic carbon flux should be reassessed using adequate samples (i.e. at least two times in low-flow month, four times in high-flow month and one time per day during the flood period), daily water discharge and sediment loads and appropriate estimate method. Copyright © 2011 John Wiley & Sons, Ltd.

Parameter sensitivity of the Distributed Hydrology-Soil-Vegetation Model (DHSVM) was studied in two contrasting environments: (1) Pang Khum Experimental Watershed (PKEW) in tropical northern Thailand; and (2) Cedar River basin (CRB) in Washington State of the temperate US Pacific Northwest. The analysis shows that for both basins, the most sensitive soil parameters were porosity, lateral saturated hydraulic conductivity, and the exponential decrease rate of lateral saturated hydraulic conductivity with soil depth. The most sensitive vegetation parameters were leaf area index, vegetation height, vapour pressure deficit, minimum stomatal resistance (for both grassland and forest scenarios), hemisphere fractional coverage, overstory fractional coverage, and trunk space (for the forest scenario only). Parameter sensitivity was basin-specific, with the humid, temperate CRB being more influenced by vegetation parameters, while tropical PKEW was more influenced by soil properties. Increases and decreases in parameter values resulted in opposite and unequal changes in bias and root mean square error (RMSE), indicating the non-linearity of physical process represented in the hydrological model. Copyright © 2011 John Wiley & Sons, Ltd.

The distributed hydrology soil vegetation model (DHSVM) is applied in the 107 km2 Nam Mae Rim watershed (NMRW) in northern Thailand. Simulations using land cover scenarios for 1989 and 2002, extreme deforestation, and forest, each run with and without roads, show that roads have very small effects on the mean water fluxes, but significantly increase peak flows for all land cover scenarios. The magnitude of the road effect on peak flow depends on the land cover context in which the roads are placed. Roads have the smallest effect on peaks within the extreme deforestation scenario, and affect mainly the smallest peaks in the two homogeneous land cover scenarios (forest and extreme deforestation). Roads have the largest effect on peaks within the heterogeneous landscapes of the two historical land cover scenarios (1989 and 2002). This result indicates that, by conveying surface flow quickly to the stream, roads within a fragmented landscape are especially important in converting greater amounts of overland flow into higher peak flows. Without roads, the patchy land cover pattern buffers the impacts of the scattered overland flow source areas and limits increases in peak flows. The combined effects of land cover and roads are examined in comparison with forest. The 1989 and 2002 land cover patterns result in relatively small changes in the mean annual water balance, but with some significant changes in the seasonal distribution of fluxes. Streamflow is lower in the late wet season and higher in the dry season for the 1989 and 2002 scenarios compared with forest. Conversion from forest to any of the other land cover scenarios results in higher dry season evapotranspiration (ET) and lower wet season ET. The extreme deforestation scenario results in much lower wet season ET and much higher annual streamflow, but little change in dry season streamflow, in comparison with forest. Copyright © 2008 John Wiley & Sons, Ltd.

Hydrological responses in a zero-order basin (ZOB), a portion of whose discharge emerged via preferential flow through soil pipes, were examined over a 2-year period in Peninsular Malaysia to elucidate primary stormflow generation processes. Silicon (Si) and specific conductance (EC) in various runoff components were also measured to identify their sources. ZOB flow response was dependent on antecedent precipitation amount; runoff increased linearly with precipitation during events >20 mm in relatively wet antecedent moisture conditions. Runoff derived from direct precipitation falling onto saturated areas accounted for

[Extract] Recent flooding in Asia has drawn worldwide attention (DFO, 2012). Floods in Pakistan in late July 2010 inundated one-fifth of the country, killed nearly 2000 people, and disrupted the livelihoods of about 20 million others. Heavy rains causing flooding throughout the 2010 monsoon period killed several thousand in China, many from rain-associated landslides. By the end of August, more than 15 million people had been evacuated from flood and landslide danger zones. Again in 2011, widespread floods caused catastrophic damage in nearly all Southeast Asian countries, including Thailand, Cambodia, Laos, Malaysia, Myanmar, and the Philippines (DFO, 2012).

One difficulty in modelling sediment transport on unpaved road surfaces stems from the inability of physically based models to simulate the flush of loose surface material that is deposited on the road surface prior to the onset of a storm. This work builds upon a prior modelling methodology (referred to as dynamic erodibility) to simulate time-varying sediment transport on unpaved mountain roads by loosely coupling a continuous, exponential decay disturbance model with the erosion algorithm in KINEROS2. The method is tested against sediment transport time-series observed on small-scale rainfall simulation plots for various slope, antecedent soil wetness and pre-storm sediment availability conditions. The new method generally improves prediction errors of total sediment output, peak sediment output and fit of the sediment output time-series. However, for some validation events, the method fails to simulate high initial sediment spikes. This limitation may be a side-effect of using data from small-scale plots, but also may signify the need for additional calibration of the disturbance model subcomponent with data from surfaces having a greater variety of pre-event surface material. Nevertheless, this road erosion modelling approach provides a realistic ‘description’ of time-varying sediment transport, which is controlled both by the baseline erodibility of the underlying road surface, and importantly, by the removal of a loose, surficial sediment layer by overland flow. Copyright  2002 John Wiley & Sons, Ltd.

The event- and physics-based KINEROS2 runoff/erosion model for predicting overland flow generation and sediment production was applied to unpaved mountain roads. Field rainfall simulations conducted in northern Thailand provided independent data for model calibration and validation. Validation shows that KINEROS2 can be parameterized to simulate total discharge, sediment transport and sediment concentration on small-scale road plots, for a range of slopes, during simulated rainfall events. The KINEROS2 model, however, did not accurately predict time-dependent changes in sediment output and concentration. In particular, early flush peaks and the temporal decay in sediment output were not predicted, owing to the inability of KINEROS2 to model removal of a surface sediment layer of finite depth. After 15–20 min, sediment transport declines as the supply of loose superficial material becomes depleted. Modelled erosion response was improved by allowing road erodibility to vary during an event. Changing the model values of erosion detachment parameters in response to changes in surface sediment availability improved model accuracy of predicted sediment transport by 30–40%. A predictive relationship between road erodibility ‘states’ and road surface sediment depth is presented. This relationship allows implementation of the dynamic erodibility (DE) method to events where pre-storm sediment depth can be estimated (e.g., from traffic usage variables). Copyright © 2001 John Wiley & Sons, Ltd.

Two indirect methods are used to detect evidence of intercepted subsurface flow (ISSF) by the road prism in the Pang Khum Experimental Watershed (PKEW) in northern Thailand. During the 12-month study period we failed to observe a soil moisture change that corresponds with ISSF being generated by the water table rising above the road surface. In support of the soil moisture data, δ 18 O signatures of rain water, road runoff, and stream water (a proxy for soil water) suggest Horton overland flow (HOF) generated on the road surface, not ISSF, is the dominant source of observed road runoff during typical rainfall events in the study area. This finding, which is contrary to the ISSF-dominant road runoff regime found typically on unpaved mountain roads in the US Pacific NW, suggests that the use of a HOF-based model to simulate runoff and sediment transport on unpaved roads in PKEW provides not only lower bound estimates of these processes, but realistic approximations for typical events.