Humid Catchments Research Articles

High spatial resolution simulation of profile soil moisture by assimilating multi-source remote-sensed information into a distributed hydrological model

Profile soil moisture (PSM), which represents soil moisture content over the whole soil layer depth, is a key variable to control plant growth, biological interactions, and streamflow generation, and its information plays a crucial role in hydrological analysis and agricultural water management. Recent studies have assimilated multi-source satellite PSM information into hydrological modelling to more accurately estimate real PSM. However, the PSM estimated from these studies are normally at coarse spatial resolution (i.e., larger than 25 × 25 km2). In this study, the high-resolution (1 × 1 km2) PSM are generated by assimilating multiple remote-sensed PSM data of coarse resolution into the Digital Elevation Model (DEM) based distributed rainfall-runoff model (DDRM) in three catchments (two humid catchments and one semiarid catchment) in China with the grid scale of 1 × 1 km2. The remote-sensed PSM data are pre-processed from two remote-sensed surface soil moisture datasets, i.e. the multi-satellite-retrieved soil moisture dataset released by the Europe Space Agency Climate Change Initiative (ESA CCI), and the soil moisture product from the Soil Moisture Active Passive (SMAP) satellite. The influence of remote-sensed datasets selection schemes (i.e. only ESA CCI, only SMAP, and ESA CCI and SMAP combined) on assimilation results are investigated. In the assimilation process, two updating schemes are considered, one is to only update the DDRM’s state variable, i.e. PSM, and the other updating both the parameters and PSM variable of the DDRM. Thus, six assimilation scenarios are set in the study, whose performances are compared with the DDRM without assimilation for different time periods, including the whole period, the dormant period and the growing period. Results indicate that in any periods, for any of remote-sensed datasets used for assimilation, either the state update or the parameter-state update can improve the accuracy of high-resolution (1 × 1 km2) PSM simulations by the DDRM. Besides, assimilating the SMAP PSM dataset into the DDRM has the potential to improve streamflow simulations for the three catchments. This study has shown that, by assimilating multi-source remote-sensed PSM into a high spatial resolution distributed hydrological model, i.e. DDRM, estimation of PSM can be improved over both the original remote-sensed PSM and the DDRM-simulated PSM.

Tectonically conditioned record of continental interior paleoclimate during the Carnian Pluvial Episode: The Upper Triassic Los Rastros Formation, Argentina

Abstract Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto–Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ18Ocarb –11.6‰ and –15.7‰ (χ average –13.1‰; 1σ = 1.6) and δ13Ccarb –2.6‰ to –8.0‰ (χ average –5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Models estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

Comparing Three Hydrological Models for Flash Flood Simulations in 13 Humid and Semi-humid Mountainous Catchments

Flash flood disaster ranks top among all the natural hazards across the world due to its high frequency, severity and fatality. However, flash flood simulation is still challenging in small and medium-sized catchments with complex orography, flashy hydrological responses and poor observations. Three distributed hydrological models, i.e., TOPModel, HEC and CNFF, are selected to simulate flash floods in seven humid and six semi-humid catchments in China, with consideration of water balance (RER), peak flow rate (REQ) and its occurrence time (TP), hydrograph variation (SNSE) and model uncertainty. Influences of five catchment attributes are further investigated on individual model performances. All three models perform satisfactorily in humid catchments, but less satisfactorily in semi-humid catchments. Water balance is well obtained by CNFF, followed by HEC and TOPModel. Peak flow rate and its occurrence time are most accurately captured by CNFF and HEC, respectively. Hydrograph variations are well reproduced by HEC and CNFF. TOPModel performs well for picking peak flow and hydrograph variation in humid catchments. Uncertainty interval is narrowest for HEC with average relative interval length at 95% confidence level being 0.78 ~ 2.53. Most observations are bracketed by uncertainty intervals for TOPModel (64.79% ~ 91.91% of total). Three model performance indices (i.e., RER, REQ, and SNSE) are mainly affected by drainage area and forest ratio across humid and semi-humid catchments, while TP performance is mainly affected by mean slope in humid catchments.

Atmospheric dynamics of extreme discharge events from 1979 to 2016 in the southern Central Andes

During the South-American Monsoon season, deep convective systems occur at the eastern flank of the Central Andes leading to heavy rainfall and flooding. We investigate the large- and meso-scale atmospheric dynamics associated with extreme discharge events (> 99.9th percentile) observed in two major river catchments meridionally stretching from humid to semi-arid conditions in the southern Central Andes. Based on daily gauge time series and ERA-Interim reanalysis, we made the following three key observations: (1) for the period 1940–2016 daily discharge exhibits more pronounced variability in the southern, semi-arid than in the northern, humid catchments. This is due to a smaller ratio of discharge magnitudes between intermediate (0.2 year return period) and rare events (20 year return period) in the semi-arid compared to the humid areas; (2) The climatological composites of the 40 largest discharge events showed characteristic atmospheric features of cold surges based on 5-day time-lagged sequences of geopotential height at different levels in the troposphere; (3) A subjective classification revealed that 80% of the 40 largest discharge events are mainly associated with the north-northeastward migration of frontal systems and 2/3 of these are cold fronts, i.e. cold surges. This work highlights the importance of cold surges and their related atmospheric processes for the generation of heavy rainfall events and floods in the southern Central Andes.

Using stable isotopes to estimate young water fractions in a heavily regulated, tropical lowland river basin

AbstractThe young water fraction of streamflow (Fyw), an important hydrological variable, has been calculated for the first time, for a monsoon‐fed coastal catchment in northern Vietnam. Oxygen stable isotopes (δ18O) from six river sites in the Day River Basin (DRB) were analysed monthly, between January 2015 and December 2018. River δ18O signatures showed sine wave variability, reflecting the amount effect and tropical (dry‐rainy) seasonality of the region. The δ18O composition of precipitation ranged from −12.67 to +1.68‰, with a mean value of −5.14‰, and in‐streamflow signatures ranged from −11.63 to −1.37‰ with a mean of −5.02‰. Fractions of young water (Fyw) were calculated from the unweighted and flow‐weighted δ18O composition of samples. Unweighted Fyw ranged between 29 ± 8% and 82 ± 21% with a mean value of 51 ± 19%, and was not significantly different from flow‐weighted Fyw (range between 33 ± 25% and 92 ± 73%, mean 52 ± 36%). Both unweighted and flow‐weighted Fyw were highest in the middle of stream and lowest in downstream sites, capturing the impacts of landuse changes, hydrology and human activities in the catchment. Our calculations imply that more than a half of rainwater reaches the DRB river mainstream within the first 3 months. The Fyw is much higher than the global average (of one‐third) and insensitive to discharge due to the combination of a humid catchment with high rainfall, low storage capacity, flat landscape and an intensive drainage system in the DRB. Also the low discharge sensitivity of Fyw in the DRB implies that the regional hydrology is severely altered by humans.

FACILITATE DECISION MAKING OF OPTIMUM UTILIZATION URBAN ROOFTOP RWH, CASE STUDY KOLKATA METROPOLITAN AREA

The study is done for assessment of Rooftop Rain Water Harvesting (RRWH) for non potable uses in a humid urban catchment. In this study, an user response survey was conducted, with 390 sample size, in five types of building uses; Residential, Educational, Medical, Institutional and mixed use Commercial, with variable roof sizes and situated in four different zones of KMA, having wide variation in piped water supply. A database of 32 years of daily rainfall data has been analyzed , in order to find out demand for different end uses for various building, supply from roof runoff, demand supply ratio, priority of different socio-economic factors for each type of building using AHP analysis, user’s opinion on choice of end-use using regression analysis and finally developed a DSS model. Analysis also revealed that the highest acceptance of RRWH are in favor of the Medical uses building, the lowest being mix-Commercial building. Further factors like toilet flushing is found to be most potential end use options, followed by landscaping and cleaning. The regression model clearly show that the variables like ground condition, scale of development, degree of contact, storey’s of building and water scarcity are key to decision making. KEYWORDS : Roof top Rain Water Harvesting (RRWH), non potable use, Decision support system (DSS), Analytical Hierarchy Process (AHP), End-use potential, Urban Local Bodies (ULB).

Assessing the Steady‐State Assumption in Water Balance Calculation Across Global Catchments

AbstractIt has long been assumed that over a sufficiently long period of time, changes in catchment water storage (ΔS) are a relatively minor term compared to other fluxes and can be neglected in the catchment water balance equation. However, the validity of this fundamental assumption has rarely been tested, and the associated uncertainties in water balance calculations remain unknown. Here, we use long‐term (1982–2011) observations of monthly streamflow (Q) and precipitation (P) for 1,057 global unimpaired catchments, combined with four independent evapotranspiration (E) estimates to infer ΔS and to provide a global assessment of the steady‐state assumption in catchment water balance calculations. Results show that when the threshold for steady state is set to 5% of the mean monthly P, ~70% of the catchments attain steady state within 10 years while ~6% of the catchments fail to reach a steady state even after 30 years. The time needed for a catchment to reach steady state (τs) shows a close relationship with climatic aridity and vegetation coverage, with arid/semiarid and sparsely vegetated catchments generally having a longer τs. Additionally, increasing snowfall fraction also increases τs. The imbalance (ewb) caused by ignoring ΔS decreases as averaging period for water balance calculations increases as expected. For a typical 10‐year averaging period, ewb accounts for ~7% of P in arid, but that decreases to ~3% of P in humid catchments. These results suggest that catchment properties should be considered when applying the steady‐state assumption and call for caution when ignoring ΔS in arid/semiarid regions.

Attribution of Evapotranspiration Changes in Humid Regions of China from 1982 to 2016

AbstractThis study quantifies the influences of climate variation and vegetation greening on the changes in evapotranspiration (ET) over 110 humid catchments of China from 1982 to 2016. A process‐based model considering climate and vegetation changes is employed to estimate ET. The simulated ET compared well with the observed values based on the flux measurements and water balance equation, with the coefficients of determination above 0.62. During the study period, environmental change of the study area was characterized by the increases in air temperature and leaf area index, that is, climate warming and vegetation greening. Annual ET increased at 104 catchments with the trends ranging from 0.01 to 6.2 mm yr−2. The numerical experiment approach and sensitivity method are used to attribute ET changes to climate variation and vegetation greening. The results show that the contributions of climate variation to ET changes estimated by the two methods are 90.6% and 91.3%, respectively, whereas the contributions of vegetation greening to ET changes are only 9.4% and 8.7%, indicating that climate variation controls ET changes in the study area. However, vegetation greening alters the proportions of ET components, which contributes 59.0%–62.0% to the increase in the ratio of transpiration to ET. This study quantifies the roles of climate and vegetation changes in the dynamics of ET, suggesting that more concerns should be paid on the interactions between ecological processes and hydrological cycle in humid regions.

Effects of forest cover change on catchment evapotranspiration variation in China

AbstractSeveral large‐scale revegetation programs in China have resulted in significant changes in forest cover (ΔF) during the past three decades. As forests have important effects on catchment hydrology, evaluating the effects of ΔF on hydrology is essential. Using data from 44 catchments across China, this study derived a rational analytical equation to link changes in actual evapotranspiration (ΔET) with those in F within the Budyko framework, and further quantified the effects of ΔF on ET variation during 1976–2015. The elasticity of ET to ΔF was found to be the greatest in the dry catchments in northwest China, followed by the humid catchments in south China, and was the smallest in the subhumid and semiarid catchments in north China. F averaged across all the catchments has increased, and has further led to the increase in ET. The F‐ET relationship has become more prominent in the recent decade (2006–2015), with 68.0% of the catchments showing an average increase in F of 4.5% and a resultant average increase in ET of 8.2% compared with the baseline period (1976–1985). These results are helpful for quantitative assessment of hydrological responses to afforestation, especially in water‐limited regions.

A Novel Method of Quantifying Catchment‐Wide Average Peak Propagation Speed in Hillslopes: Fast Hillslope Responses are Detected During Annual Floods in a Steep Humid Catchment

AbstractThe lag time between rainfall input and discharge output—that is, the response speed of a catchment—is a key parameter for flood prediction but is difficult to quantify. Here we propose a new method to quantify catchment‐wide average peak propagation speed in hillslopes based on measurements conducted in a steep 4.5‐km2 catchment. The peak lag time of the catchment was partitioned into lag times in the hillslope and channel. Peak propagation speed was calculated from flow path length divided by lag time. We verified the results and evaluated the applicability and uncertainty of this method. This method can be applied during intense storms with identifiable peak relationships between rainfall and discharge. The effect of spatial variation in precipitation was insignificant in the study catchment. The increase in rainfall intensity and the rainfall accumulated prior to peak rainfall increased peak propagation speed. The method used to characterize flow path length from distributed values introduced some uncertainty. We calculated modal lengths using linear and logarithmic scales and various bin widths. These calculations resulted in small differences in modal channel length but led to more than tenfold differences in modal hillslope length. Thus, uncertainties for peak propagation speed across all hillslopes varied by one order of magnitude. Hillslope lag times were less than 2 min and peak propagation speed in the hillslopes ranged from 0.04 to 4.3 m/s for floods with recurrence intervals of one to several years. The proposed method and calculated data should improve prediction accuracy for extreme floods.

Applicability of Hydrological Models for Flash Flood Simulation in Small Catchments of Hilly Area in China

Abstract Flash flood in small catchments of hilly area is an extremely complicated nonlinear process affected by catchment properties and rainfall spatio-temporal variation characteristics including many physical-geographical factors, and thus accurate simulation of flash flood is very difficult. Given the fact that hundreds of hydrological models are available in the literature, how to choose a suitable hydrological model remains an unsolved task. In this paper, we selected five widely used hydrological models including three lumped hydrologic models, a semi-distributed hydrological model and a distributed hydrological model for flash flood simulation, and studied their applicability in fourteen typical catchments in hilly areas across China. The results show that the HEC-HMS distributed hydrological model outperforms the other models and is suitable to simulate the flash floods caused by highly intense rainfall. The Dahuofang model (lumped) has higher precision in peak runoff time simulation. However, its performance is quite poor on the flood volume simulation in the small catchments characterized by intense vegetation coverage and highly developed stream network. The Antecedent precipitation index and Xinanjiang models (lumped) can obtain good simulation results in small humid catchments as long as long-term historical precipitation and runoff data are provided. The TOPMODEL also shows good performance in small humid catchments, but it is unable to simulate the flash floods characterized by the rapid rise and recession. Our results could be very beneficial in practice, since these provide a solid foundation in the selection of hydrological model for flash flood simulation in small catchments in hilly area.

Multiobjective sensitivity analysis and model parameterization approach for coupled streamflow and groundwater table depth simulations using SHETRAN in a wet humid tropical catchment

Physically based Distributed Hydrological Models (PDHM) are complex and exhibits extreme challenges for simulating the dynamics of coupled streamflow and groundwater table depths. The present work assesses the capabilities of a PDHM, SHETRAN in capturing streamflow and depth to groundwater table to represent the hydrologic processes within a wet tropical humid catchment. The most significant model parameters for the coupled simulations of streamflow and groundwater table depths are assessed through a multiobjective sensitivity analysis using Morris screening method and local sensitivity analysis. Horizontal and vertical soil saturated hydraulic conductivities, specific storage, as well as Actual/Potential evapotranspiration and Strickler overland coefficient, are highly sensitive for both the model responses considered. In order to arrive at the optimal model parameters for simulating depth to water table and streamflow, the effect of three model parametrization approaches were analyzed: with no spatial variation in soil properties, elevation based variation of only middle and bottom soil layer saturated hydraulic conductivity and elevation based variation of horizontal (kx), vertical (kz) soil saturated hydraulic conductivities and specific storage (Ss). Results indicate that for the studied watershed, the elevation based variation in kx, kz, and Ss resulted in a better prediction of depth to water table variations (in terms of correlation coefficient) with no significant improvements in streamflow simulation compared to the other two parameterization cases. The study suggests incorporating preferential flows in SHETRAN model to improve prediction of the water table dynamics in catchments belonging to the humid tropical region.

Utilizing Satellite Surface Soil Moisture Data in Calibrating a Distributed Hydrological Model Applied in Humid Regions Through a Multi-Objective Bayesian Hierarchical Framework

The traditional calibration objective of hydrological models is to optimize streamflow simulations. To identify the value of satellite soil moisture data in calibrating hydrological models, a new objective of optimizing soil moisture simulations has been added to bring in satellite data. However, it leads to problems: (i) how to consider the trade-off between various objectives; (ii) how to consider the uncertainty these satellite data bring in. Among existing methods, the multi-objective Bayesian calibration framework has the potential to solve both problems but is more suitable for lumped models since it can only deal with constant variances (in time and space) of model residuals. In this study, to investigate the utilization of a soil moisture product from the Soil Moisture Active Passive (SMAP) satellite in calibrating a distributed hydrological model, the DEM (Digital Elevation Model) -based Distributed Rainfall-Runoff Model (DDRM), a multi-objective Bayesian hierarchical framework is employed in two humid catchments of southwestern China. This hierarchical framework is superior to the non-hierarchical framework when applied to distributed models since it considers the spatial and temporal residual heteroscedasticity of distributed model simulations. Taking the streamflow-based single objective calibration as the benchmark, results of adding satellite soil moisture data in calibration show that (i) there is less uncertainty in streamflow simulations and better performance of soil moisture simulations either in time and space; (ii) streamflow simulations are largely affected, while soil moisture simulations are slightly affected by weights of objectives. Overall, the introduction of satellite soil moisture data in addition to observed streamflow in calibration and putting more weights on the streamflow calibration objective lead to better hydrological performance. The multi-objective Bayesian hierarchical framework implemented here successfully provides insights into the value of satellite soil moisture data in distributed model calibration.

Cosmogenic 10Be in river sediment: where grain size matters and why

Abstract. Concentrations of in-situ-produced cosmogenic 10Be in river sediment are widely used to estimate catchment-average denudation rates. Typically, the 10Be concentrations are measured in the sand fraction of river sediment. However, the grain size of bedload sediment in most bedrock rivers covers a much wider range. Where 10Be concentrations depend on grain size, denudation rate estimates based on the sand fraction alone are potentially biased. To date, knowledge about catchment attributes that may induce grain-size-dependent 10Be concentrations is incomplete or has only been investigated in modelling studies. Here we present an empirical study on the occurrence of grain-size-dependent 10Be concentrations and the potential controls of hillslope angle, precipitation, lithology, and abrasion. We first conducted a study focusing on the sole effect of precipitation in four granitic catchments located on a climate gradient in the Chilean Coastal Cordillera. We found that observed grain size dependencies of 10Be concentrations in the most-arid and most-humid catchments could be explained by the effect of precipitation on both the scouring depth of erosion processes and the depth of the mixed soil layer. Analysis of a global dataset of published 10Be concentrations in different grain sizes (n=73 catchments) – comprising catchments with contrasting hillslope angles, climate, lithology, and catchment size – revealed a similar pattern. Lower 10Be concentrations in coarse grains (defined as “negative grain size dependency”) emerge frequently in catchments which likely have thin soil and where deep-seated erosion processes (e.g. landslides) excavate grains over a larger depth interval. These catchments include steep (> 25∘) and humid catchments (> 2000 mm yr−1). Furthermore, we found that an additional cause of negative grain size dependencies may emerge in large catchments with weak lithologies and long sediment travel distances (> 2300–7000 m, depending on lithology) where abrasion may lead to a grain size distribution that is not representative for the entire catchment. The results of this study can be used to evaluate whether catchment-average denudation rates are likely to be biased in particular catchments.

Is There a Baseflow Budyko Curve?

AbstractThere is no general theory to explain differences in baseflow between catchments, despite evidence that it is mainly controlled by climate and landscape. One hypothesis is that baseflow fraction (the ratio between baseflow and precipitation) can be primarily attributed to the aridity index (the ratio between potential evapotranspiration and precipitation), that is, that there is a “baseflow Budyko curve.” Comparing catchment data from the United States and the United Kingdom shows, however, that aridity is not always a good predictor of baseflow fraction. We use the revised Ponce‐Shetty annual water balance model to show that there is no single “baseflow Budyko curve” but rather a continuum of curves emerging from a more universal model that incorporates both climate and landscape factors. In humid catchments, baseflow fraction is highly variable due to variations in a catchment's wetting potential, a parameter that describes catchment storage capacity. In arid catchments, vaporization limits baseflow generation, which leads to lower variability in baseflow fraction. Generally, when the magnitude of precipitation is important, the aridity index only partly explains baseflow response. Adapting the model to explain variability of the baseflow index (the ratio between baseflow and total streamflow) shows that the aridity index is generally a poor predictor of baseflow index. While the wetting potentials and other parameters are obtained by fitting the Ponce‐Shetty model to annual catchment data, their links to physical properties remain to be explored. This currently limits the model's applicability to gauged catchments with sufficiently long records.

Evaluating Runoff Generation in a Humid Bamboo Watershed Using Isotopic and Hydrochemical Tracer

AbstractMost of the studies in humid catchments have discovered a similar conclusion that runoff generation was dominated by overland flow and interflow. However, the Hemuqiao watershed with long-t...

Suitability of Satellite-Based Precipitation Products for Water Balance Simulations Using Multiple Observations in a Humid Catchment

This study assesses the suitability of five popular satellite-based precipitation products in modeling water balance in a humid region of China during the period 1998–2012. The satellite-based precipitation products show similar spatial patterns with varying degrees of overestimation or underestimation, compared with the gauged precipitation. A distributed hydrological model is used to evaluate the suitability of satellite-based precipitation products in simulating streamflow, evapotranspiration and soil moisture. The simulations of streamflow and evapotranspiration forced by the MSWEP precipitation perform best among the five satellite-based precipitation products, where the Kling-Gupta efficiency (KGE) between the simulated and observed streamflow ranges from 0.75 to 0.91, and the KGE between the simulated and observed evapotranspiration ranges from 0.46 to 0.61. However, the KGE between the simulated and observed soil moisture is negative, indicating that the performance of soil moisture simulation forced by satellite-based precipitation is poor. In addition, this study finds the spatial pattern of simulated streamflow is dominated by the distribution of precipitation, whereas the distribution of evapotranspiration and soil moisture is controlled by the parameters of the hydrological model. This study is useful for the improvement of hydrological modeling based on remote sensing and the monitoring of regional water resources.

Identifying time-varying hydrological model parameters to improve simulation efficiency by the ensemble Kalman filter: A joint assimilation of streamflow and actual evapotranspiration

Hydrological model parameters are essential for model simulation, which may vary with time owing to climatic variations and human activities. As a result, the implementation of stationary parameters may lead to inaccurate streamflow simulation. Actual evapotranspiration (ET) is a crucial component of hydrological cycles with an important influence on regional climate characteristics. Our aim was to investigate an appropriate way to estimate time-varying parameters using streamflow and ET observations, and to explore the how the simulation efficiency would change when ET data were added into assimilation. Data assimilation techniques can automatically adjust hydrological model parameters according to changing conditions. The ensemble Kalman filter (EnKF) technique was used to assimilate observations into a two-parameter monthly water balance model. Four data assimilation schemes were designed and applied in a synthetic experiment and 173 catchments in USA, including: (1) sole assimilation of streamflow to update both parameters, (2) joint assimilation of streamflow and ET to simultaneously update parameters, (3) sole assimilation of ET first to update parameter C (measuring evapotranspiration), and then subsequently joint assimilation of streamflow and ET to update parameter SC (measuring water storage capacity), and (4) sole assimilation of ET to update parameter C, and then sole assimilation of streamflow to update parameter SC. The four schemes were compared in terms of deterministic and probabilistic model performances, and model parameter correlations. The results indicated that: (1) the estimation of parameter C can be improved by assimilating ET observations into the model, and the time-varying model parameters obtained by adding ET data into assimilation led to a significant enhancement in deterministic streamflow and ET prediction, (2) among the three joint assimilation schemes, the deterministic model simulation performances were similar, while the ensemble predictions were more reliable in scheme 2. The joint assimilation of streamflow and ET to update both parameters in one step, i.e., scheme 2, outperformed the other two schemes in parameter SC estimation and model performance, (3) the ability to enhance simulation accuracy and to weaken correlations between parameters was more significant in humid catchments. The joint assimilation was helpful for the application of multiple sources of information into a hydrological simulation.

Long-Term Suspended Sediment Concentrations and Loads from a Relatively Undisturbed Agroforested Catchment in the Northwest of the Iberian Peninsula

The suspended sediment dynamics in small catchments are difficult to estimate accurately because they result from the coupling of complex processes occurring at different scales. In this study, the dynamics of suspended sediment concentrations (SSC) and loads were assessed in an agroforested humid catchment in NW Spain, based on a long-term rainfall, discharge and suspended sediment dataset (12 hydrological years) from high-frequency monitoring. The results highlight the episodic nature of sediment transport in the study area, given that about 78% of SS was exported over 10% of the study period. The SS transport was related to runoff generation and flooding, although sediment availability also played an important role in SS transport. The SS load was mainly driven by high-magnitude rainfall events, while intense rainfall episodes generated high SSC peaks. The mean annual suspended sediment yield was relatively low from a quantitative stand point (10 Mg km−2 y−1); however, during 11% of the monitoring, SS concentrations exceeding the threshold threatened surface water quality (Freshwater Fish Directive 78/659/EEC and Directive 75/440/EEC), mainly during runoff events, indicating the need to adopt management practices in order to reduce or mitigate sediment loss during such episodes.

Spatio-temporal controls of dissolved organic carbon stream water concentrations

Elevated concentration and high variance of dissolved organic carbon (DOC) in surface waters are a challenge for the production of drinking water. Past studies have indicated a dominant role of wetlands in DOC mobilization, but have mainly been focused on boreal and oceanic catchments. Here we analyze the observational DOC time series from 89 temperate humid catchments which drain into German drinking water reservoirs. We characterize the DOC concentration median and variability and utilize partial least squares regression in order to quantify the relation to catchment characteristics such as land use, climate, and topography. We found that the long-term median DOC concentration in the catchment is well predicted by the 90th percentile of the distribution of the topographic wetness index (0.9P TWI) over the entire catchment area. The 0.9P TWI can be directly connected to the abundance of riparian wetlands in the catchments. DOC concentration variability (represented as the ratio of the interquartile range and the median concentrations) was also found to be well predictable. Concentration variability was highest in cold and wet catchments with a high 0.9P TWI. Here we also found stronger correlations between DOC concentrations and discharge, with positive concentration-discharge patterns. Catchments with elevated DOC-concentration variance also exhibited the most severe long-term increases in concentrations. Our results thus indicate that, in temperate climates, riparian wetlands can be the dominant source zones of DOC and control the hydrological mobilization and potentially also the spatial difference in long-term concentration trends observed in surface waters. We conclude that the dominance of topography and climatic conditions in controlling spatio-temporal patterns in DOC concentrations leads to very limited management options.