Quantifying the impact of land-use changes at the event and seasonal time scale using a process-oriented catchment model

International audienceFor optimal protection and integrated management of water resources, it is essential to quantify the impact of land-use change on hydrological regimes at various scales. Using the process-based catchment model TACD (tracer aided catchment model, distributed) two land-use scenarios were analysed for the rural and mountainous Dreisam basin (258 km2): (i) an increase in urban area from 2.5% to 5%) and (2) a change in a natural land-use to a different kind of forest. The first scenario was executed using the land-use change modelling kit LUCK, which takes into account the topology of land-use patterns in their true positions. The TACD model simulated all hydrological processes both spatially and temporally (200 m x 200 m grid, hourly mode). For this study, physically-based modules for interception and evapotranspiration (Penman and Monteith approach) were introduced. The model was applied to the Dreisam basin with minimal calibration. Both an independent validation period and discharge in four nested sub-basins were modelled well without recalibration. Evapotranspiration patterns were simulated, successfully, both temporally and spatially. Increased urbanisation had an insignificant effect on the modelled single events and on the yearly water balance. Simulations of discharge from forest assuming natural land-use conditions indicated an increase in transpiration, a decrease in groundwater recharge and, consequently, in groundwater discharge (?15%), in surface water discharge (?4%), and in flood peaks (?22.7% and ?7.3% for convective and advective floods, respectively). Land-use impact was also investigated by applying rainfall scenarios of different durations (12, 24, 48, and 72 hours), magnitudes (recurrence intervals of 1, 5, and 10 years) and distributions of rainfall intensity, i.e. maximum intensity at the beginning, middle or end of the event. Clearly, the intensity distribution has a greater influence on the simulated events than different land use scenarios. This indicated the importance of careful determination of the temporal intensity distribution for flood peak predictions. The use of the process-based model enabled analysis of the altered composition of internal runoff components. This demonstrated the potentially significant local effects of land-use change on flood runoff and water quality. Keywords: land-use change, predictions, process-based catchment modelling, flood modelling, evapotranspiration modelling TACD model, LUC

Significance of spatial variability in precipitation for process-oriented modelling: results from two nested catchments using radar and ground station data

International audienceThe importance of considering the spatial distribution of rainfall for process-oriented hydrological modelling is well-known. However, the application of rainfall radar data to provide such detailed spatial resolution is still under debate. In this study the process-oriented TACD (Tracer Aided Catchment model, Distributed) model had been used to investigate the effects of different spatially distributed rainfall input on simulated discharge and runoff components on an event base. TACD is fully distributed (50x50m2 raster cells) and was applied on an hourly base. As model input rainfall data from up to 7 ground stations and high resolution rainfall radar data from operational C-band radar were used. For seven rainfall events the discharge simulations were investigated in further detail for the mountainous Brugga catchment (40km2) and the St. Wilhelmer Talbach (15.2km2) sub-basin, which are located in the Southern Black Forest Mountains, south-west Germany. The significance of spatial variable precipitation data was clearly demonstrated. Dependent on event characteristics, localized rain cells were occasionally poorly captured even by a dense ground station network, and this resulted in inadequate model results. For such events, radar data can provide better input data. However, an extensive data adjustment using ground station data is required. For this purpose a method was developed that considers the temporal variability in rainfall intensity in high temporal resolution in combination with the total rainfall amount of both data sets. The use of the distributed catchment model allowed further insights into spatially variable impacts of different rainfall estimates. Impacts for discharge predictions are the largest in areas that are dominated by the production of fast runoff components. The improvements for distributed runoff simulation using high resolution rainfall radar input data are strongly dependent on the investigated scale, the event characteristics and the existing monitoring network

Characterisation of stable isotopes to identify residence times and runoff components in two meso-scale catchments in the Abay/Upper Blue Nile basin, Ethiopia

Measurements of the stable isotopes oxygen-18 (¹⁸O) and deuterium (²H) were carried out in two meso-scale catchments, Chemoga (358 km²) and Jedeb (296 km²) south of Lake Tana, Abay/Upper Blue Nile basin, Ethiopia. The region is of paramount importance for the water resources in the Nile basin, as more than 70% of total Nile water flow originates from the Ethiopian highlands. Stable isotope compositions in precipitation, spring water and streamflow were analysed (i) to characterise the spatial and temporal variations of water fluxes; (ii) to estimate the mean residence time of water using a sine wave regression approach; and (iii) to identify runoff components using classical two-component hydrograph separations on a seasonal timescale. <br><br> The results show that the isotopic composition of precipitation exhibits marked seasonal variations, which suggests different sources of moisture generation for the rainfall in the study area. The Atlantic–Indian Ocean, Congo basin, Upper White Nile and the Sudd swamps are the potential moisture source areas during the main rainy (summer) season, while the Indian–Arabian and Mediterranean Sea moisture source areas during little rain (spring) and dry (winter) seasons. The spatial variation in the isotopic composition is influenced by the amount effect as depicted by moderate coefficients of determination on a monthly timescale (<i>R</i>² varies from 0.38 to 0.68) and weak regression coefficients (<i>R</i>² varies from 0.18 to 0.58) for the altitude and temperature effects. A mean altitude effect accounting for −0.12&permil;/100 m for ¹⁸O and −0.58&permil;/100 m for ²H was discernible in precipitation isotope composition. <br><br> Results from the hydrograph separation on a seasonal timescale indicate the dominance of event water, with an average of 71 and 64% of the total runoff during the wet season in the Chemoga and Jedeb catchments, respectively. <br><br> Moreover, the stable isotope compositions of streamflow samples were damped compared to the input function of precipitation for both catchments. This damping was used to estimate mean residence times of stream water of 4.1 and 6.0 months at the Chemoga and Jedeb catchment outlets, respectively. Short mean residence times and high fractions of event water components recommend catchment management measures aiming at reduction of overland flow/soil erosion and increasing of soil water retention and recharge to enable sustainable development in these agriculturally dominated catchments

Comprehensive flood mitigation and management in the Chi River Basin, Thailand

Severe flooding of the flat downstream area of the Chi River Basin occurs frequently. This flooding is causing catastrophic loss of human lives, damage and economic loss. Effective flood management requires a broad and practical approach. Although flood disasters cannot completely be prevented, major part of potential loss of lives and damages can be reduced by comprehensive mitigation measures. In this paper, the effects of river normalisation, reservoir operation, green river (bypass), and retention have been analysed by using integrated hydrologic and hydraulic modelling. Every tributary has been simulated by a process-based hydrological model (SWAT) coupled with the 1D/2D SOBEK river routing model. Model simulation results under the design rainfall event, i.e. flood depth, flood extent, and damages for the situation with and without flood mitigation measures have been compared and evaluated to determine an optimal set of mitigation measures. The results reveal that a combination of river normalisation, reservoir operation, and green river (bypass) is most effective as it can decrease the extent of the 100-year flood event by approximately 24% and 31% for the economic damage. The results of this study will be useful for improving the present flood defence practice in the Chi River Basi

On the validity of modeling concepts for the simulation of groundwater flow in lowland peat areas – case study at the Zegveld experimental field

The groundwater flow models currently used in the western part of The Netherlands and in other similar peaty areas are thought to be a too simplified representation of the hydrological reality. One of the reasons is that, due to the schematization of the subsoil, its heterogeneity cannot be represented adequately. Moreover, the applicability of Darcy's law in these types of soils has been questioned, but this law forms the basis of most groundwater flow models. &lt;br&gt;&lt;br&gt; With the purpose of assessing the typical heterogeneity of the subsoil and to verify the applicability of Darcy's law, geo-hydrological fieldwork was completed at an experimental field within a research area in the western part of The Netherlands. The assessments were carried out for the so-called Complex Confining Layer (CCL), which is the Holocene peaty to clayey layer overlying Pleistocene sandy deposits. Borehole drilling through the CCL with a hand auger was completed and revealed the typical heterogeneous character of this layer, showing a dominance of muddy, humified peat which is alternated with fresher peat and clay. &lt;br&gt;&lt;br&gt; Slug tests were carried out to study the applicability of Darcy's law, given that previous studies suggested its non-validity for humified peat soils due to a variable horizontal hydraulic conductivity &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;h&lt;/sub&gt; with head differences. For higher humification degrees, the experiments indeed suggested a variable &lt;i&gt;K&lt;/i&gt;&lt;sub&gt;h&lt;/sub&gt;, but this appeared to be the result of the inappropriate use of steady-state formulae for transient experiments in peaty environments. The muddy peat sampled has a rather plastic nature, and the high compressibility of this material leads to transient behavior. However, using transient formulae, the slug tests conducted for different initial groundwater heads showed that there was hardly any evidence of a variation of the hydraulic conductivity with the applied head differences. Therefore, Darcy's law can be used for typical peat soils present in The Netherlands. &lt;br&gt;&lt;br&gt; The heterogeneity of the subsoil and the apparent applicability of Darcy's law were taken into account for the detailed heterogeneous model that was prepared for the research area. A MODFLOW model consisting of 13 layers in which 4 layers represent the heterogeneous CCL was set up for an average year, assuming steady-state conditions; and for the winter of 2009 to 2010, adopting transient conditions. The transient model was extended to simulate for longer periods with the objective of visualizing the flow paths through the CCL. The results from these models were compared with a 10 layer model, whereby the CCL is represented by a single layer assuming homogeneity. From the comparison of the two model types, the conclusion could be drawn that a single layer schematization of the CCL produces flowpath patterns which are not the same but still quite similar to a 4 layer representation of the CCL. However, the single layer schematization results in a considerable underestimation of the flow velocity, and subsequently a longer travel time, through the CCL. Therefore, a single layer model of the CCL seems quite appropriate to represent the general flow behavior of the shallow groundwater system, but would be inappropriate for transport modeling through the CCL

The causes of flow regime shifts in the semi-arid Hailiutu River, Northwest China

Identifying the causes (climate vs. human activities) for hydrological variability is a major challenge in hydrology. This paper examines the flow regime shifts, changes in the climatic variables such as precipitation, evaporation, temperature, and crop area in the semi-arid Hailiutu catchment in the middle section of the Yellow River by performing several statistical analyses. The Pettitt test, cumulative sum charts (CUSUM), regime shift index (RSI) method, and harmonic analysis were carried out on annual, monthly, and daily discharges. Four major shifts in the flow regime have been detected in 1968, 1986, 1992 and 2001. Characteristics of the flow regime were analyzed in the five periods: 1957–1967, 1968–1985, 1986–1991, 1992–2000, and 2001–2007. From 1957 to 1967, the flow regime reflects quasi natural conditions of the high variability and larger amplitude of 6 months periodic fluctuations. The river peak flow was reduced by the construction of two reservoirs in the period 1968–1985. In the period of 1986–1991, the river discharge further decreased due to the combined influence of river diversions and increase of groundwater extractions for irrigation. In the fourth period of 1992–2000, the river discharge reached lowest flow and variation in corresponding to a large increase in crop area. The flow regime recovered, but not yet to natural status in the fifth period of 2001–2007. Climatic factors are found not likely responsible for the changes in the flow regime, but the changes in the flow regime are corresponding well to historical land use policy changes

Implementation of a process-based catchment model in a poorly gauged, highly glacierized Himalayan headwater

The paper presents a catchment modeling approach for remote glacierized Himalayan catchments. The distributed catchment model TAC^D, which is widely based on the HBV model, was further developed for the application in highly glacierized catchments on a daily timestep and applied to the Nepalese Himalayan headwater Langtang Khola (360 km²). Low laying reference stations are taken for temperature extrapolation applying a second order polynomial function. Probability based statistical methods enable bridging data gaps in daily precipitation time series and the redistribution of cumulated precipitation sums over the previous days. Snow and ice melt was calculated in a distributed way based on the temperature-index method employing calculated daily potential sunshine durations. Different melting conditions of snow and ice and melting of ice under debris layers were considered. The spatial delineation of hydrological response units was achieved by taking topographic and physiographic information from maps and satellite images into account, and enabled to incorporate process knowledge into the model. Simulation results demonstrated that the model is able to simulate daily discharge for a period of 10 years and point glacier mass balances observed in the research area with an adequate reliability. The simple but robust data pre-processing and modeling approach enables the determination of the components of the water balance of a remote, data scarce catchment with a minimum of input data

The use of remote sensing to quantify wetland loss in the Choke Mountain range, Upper Blue Nile basin, Ethiopia

Wetlands provide multiple ecosystem services such as storing and regulating water flows and water quality, providing unique habitats to flora and fauna, and regulating micro-climatic conditions. Conversion of wetlands for agricultural use is a widespread practice in Ethiopia, particularly in the southwestern part where wetlands cover large areas. Although there are many studies on land cover and land use changes in this region, comprehensive studies on wetlands are still missing. Hence, extent and rate of wetland loss at regional scales is unknown. The objective of this paper is to quantify wetland dynamics and estimate wetland loss in the Choke Mountain range (area covering 17 443 km&lt;sup&gt;2&lt;/sup&gt;) in the Upper Blue Nile basin, a key headwater region of the river Nile. Therefore, satellite remote sensing imagery of the period 1986–2005 were considered. To create images of surface reflectance that are radiometrically consistent, a combination of cross-calibration and atmospheric correction (Vogelman-DOS3) methods was used. A hybrid supervised/unsupervised classification approach was used to classify the images. Overall accuracies of 94.1% and 93.5% and Kappa Coefficients of 0.908 and 0.913 for the 1986 and 2005 imageries, respectively were obtained. The results showed that 607 km&lt;sup&gt;2&lt;/sup&gt; of seasonal wetland with low moisture and 22.4 km&lt;sup&gt;2&lt;/sup&gt; of open water are lost in the study area during the period 1986 to 2005. The current situation in the wetlands of Choke Mountain is characterized by further degradation which calls for wetland conservation and rehabilitation efforts through incorporating wetlands into watershed management plans

Ecohydrological characterization of the Nyando wetland, Lake Victoria, Kenya: A State of System (SoS) analysis

Lake Victoria floodplain wetlands have a complex hydrological setting characterized by transition from a terrestrial to an aquatic environment. A state-of-system (SoS) analysis was carried out in a papyrus dominated wetland in the Nyando River Delta, on the eastern shores of Lake Victoria, Kenya, to characterize and provide data for detailed ecohydrological studies. The objectives of the study were to: (1) determine the spatio- temporal changes in the wetland evolution and (2) analyze the main hydrological factors that have influenced wetland evolution. Multi-temporal dry-season Landsat MSS, Landsat TM and Landsat ETM+ imagery covering Nyando Wetland and its surrounding area were processed and analyzed to generate time series polygon and polyline maps of the wetland and river. Results show that the wetland increased in size from 5,925 ha in 1950 to 9,925 ha in 1973, and declined to 4,527 ha in 2008. In the last 60 years, Nyando River has migrated in a general eastward direction. Time series hydrological data (1950-2009) were statistically tested for homogeneity  using the Spearman’s rank test for linear trends, Pettit test and Standard Normal Homogeneity test (SNHT) for change point analysis, and split-record tests performed for variance (F-test) and mean (t-test). In addition, data were analyzed using descriptive statistics and frequency analyses. Statistical test results show that the hydrological data series were homogeneous. Results of change point analyses indicate that total annual rainfall in Nyando declined in 1979, while the mean annual discharge for Nyando River and Lake Victoria levels had significant upward shifts in 1961. The decadal mean discharges varied significantly over time and increased by 80% from 11.45 m3/s observed in the 1950-1961 subset, reducing by 11.4 and 21.9% in the next two decadal sub-sets, before rising by 35.0% in 1990s and dropping by 24.0% in the last decade. The decadal mean annual lake levels increased from 1134.0 to 1135.43 m in the 1951-1961 and 1962-1972 and remained above the longterm mean of 1135.0 m for 43 years since 1962 before dropping drastically by 1.4 m to an average of 1134 m/year in 2005-2009. The highest recorded lake level at Kisumu Station was 1136.2 m in 1964 after increasing by 2.5 m from 1961. Discharge data exhibit trimodal seasonal patterns, while the lake levels had two peaks. The lake levels are more sensitive to direct lake rainfall. Changes in the Nyando wetland area are linked to the seasonal and episodic flood and drought events coupled with anthropogenic activities (regulation of lake levels, modification of river including cut-off meanders, river training and construction of dykes, drainage of wetland for cultivation, settlement and livestock grazing, abstraction of water for irrigation). A combination of these hydrological and human factors is the main cause of the Nyando Wetland evolution. If the land use trend continues unabated, then the increase in papyrus losses will pose a big challenge to the ecological functioning of the wetland and its support to sustaining community livelihoods.Key words: Nyando Wetland, River, ecohydrology, Lake Victoria

Regionalising a meso-catchment scale conceptual model for river basin management in the semi-arid environment

Meso-scale catchments are often of great interest for water resources development and for development interventions aimed at uplifting rural livelihoods. However, in Sub-Saharan Africa IWRM planning in such catchments, and the basins they form part of, are often ungauged or constrained by poor data availability. Regionalisation of a hydrological model presents opportunities for prediction in ungauged basins and catchments. This study regionalises HBVx, derived from the conceptual hydrological model HBV, in the semi-arid Mzingwane Catchment, Limpopo Basin, Zimbabwe. Fifteen meso-catchments were studied, including three that were instrumented during the study. Discriminant analysis showed that the characteristics of catchments in the arid agro-ecological Region V were significantly different from those in semi-arid Region IV. Analysis of flow duration curves statistically separated sub-perennial catchments from (sub-)ephemeral catchments. Regionalised parameter sets for HBVx were derived from means of parameters from the sub-perennial catchments, the (sub-)ephemeral catchments and all catchments. The parameter sets that performed best in the regionalisation are characterised by slow infiltration with moderate/fast “overland flow”. These processes appear more extreme in more degraded catchments. This is points to benefits to be derived from conservation techniques that increase infiltration rate and from runoff farming. Faster, and possibly greater, sub-surface contribution to streamflow is expected from catchments underlain by granitic rocks. Calibration and regionalisation were more successful at the dekad (10 days) time step than when using daily or monthly data, and for the sub-perennial catchments than the (sub-)ephemeral catchments. However, none of the regionalised parameter sets yielded CNS ⩾ 0.3 for half of the catchments. The HBVx model thus does offer some assistance to river basin planning in semi-arid basins, particularly for predicting flows in ungauged catchments at longer time steps, such as for water allocation purposes. However, the model is unreliable for more ephemeral and drier catchments. Without more reliable and longer rainfall and runoff data, regionalisation in semi-arid ephemeral catchments will remain highly challengin