T.H.M. Rientjes

Rainfall Variability over Mountainous and Adjacent Lake Areas: The Case of Lake Tana Basin at the Source of the Blue Nile River

Abstract The water resource of the Blue Nile River is of key regional importance to the northeastern African countries. However, little is known about the characteristics of the rainfall in the basin. In this paper, the authors presented the space–time variability of the rainfall in the vicinity of Lake Tana, which is the source of the Blue Nile River. The analysis was based on hourly rainfall data from a network of newly installed rain gauges, and cloud temperature indices from the Meteosat Second Generation (MSG–2) Spinning Enhanced Visible and Infrared Imager (SEVIRI) satellite sensor. The spatial and temporal patterns of rainfall were examined using not only statistical techniques such as exceedance probabilities, spatial correlation structure, harmonic analysis, and fractal analysis but also marginal statistics such as mean and standard deviation. In addition, a convective index was calculated from remote sensing images to infer the spatial and temporal patterns of rainfall. Heavy rainfall is frequen...

Multiobjective training of artificial neural networks for rainfall-runoff modeling

This paper presents results on the application of various optimization algorithms for the training of artificial neural network rainfall-runoff models. Multilayered feed-forward networks for forecasting discharge from two mesoscale catchments in different climatic regions have been developed for this purpose. The performances of the multiobjective algorithms Multi Objective Shuffled Complex Evolution Metropolis–University of Arizona (MOSCEM-UA) and Nondominated Sorting Genetic Algorithm II (NSGA-II) have been compared to the single-objective Levenberg-Marquardt and Genetic Algorithm for training of these models. Performance has been evaluated by means of a number of commonly applied objective functions and also by investigating the internal weights of the networks. Additionally, the effectiveness of a new objective function called mean squared derivative error, which penalizes models for timing errors and noisy signals, has been explored. The results show that the multiobjective algorithms give competitive results compared to the single-objective ones. Performance measures and posterior weight distributions of the various algorithms suggest that multiobjective algorithms are more consistent in finding good optima than are single-objective algorithms. However, results also show that it is difficult to conclude if any of the algorithms is superior in terms of accuracy, consistency, and reliability. Besides the training algorithm, network performance is also shown to be sensitive to the choice of objective function(s), and including more than one objective function proves to be helpful in constraining the neural network training.

Multi - objective performance comparison of an artificial neural network and a conceptual rainfall - runoff model

Abstract A multi-objective comparison between an artificial neural network and the conceptual HBV rainfall—runoff model has been performed. The popular NSGA-II algorithm was used for calibration of both models. A combination of three objective functions was used to evaluate model performance. The results show that, for a small forecast lead time, the artificial neural network outperformed the HBV model on the objective functions for low and high flows, but the former was outperformed on a novel objective function related to the shape of the hydrograph. As the forecast horizon increases, the HBV model starts to outperform the ANN model on all objective functions. The main conclusion of this study is that, although the differences between the two model approaches make a straightforward and unequivocal comparison difficult, the multi-objective approach enables a more reliable evaluation of the two models than the single-objective approach.

Diurnal rainfall variability over the Upper Blue Nile Basin: a remote sensing based approach

In this study we aim to assess the diurnal cycle of rainfall across the Upper Blue Nile (UBN) basin using satellite observations from Tropical Rainfall Measuring Mission (TRMM). Seven years (2002-2008) of Precipitation Radar (PR) and TRMM Microwave Imager (TMI) data are used and analyses are based on GIS operations and simple statistical techniques. Observations from PR and TMI reveal that over most parts of the basin area, the rainfall occurrence and conditional mean rain rate are highest between midand late-afternoon (15:00-18:00 LST). Exceptions to this are the south-west and south-eastern parts of the basin area and the Lake Tana basin where midnight and early morning maxima are observed. Along the Blue Nile River gorge the rainfall occurrence and the conditional mean rain rate are highest during the night (20:00-23:00 LST). Orographic effects by large scale variation of topography, elevation and the presence of the UBN river gorge were assessed taking two transects across the basin. Along transects from north to south and from east to west results indicate increased rainfall with increase of elevation whereas areas on the windward side of the high mountain ranges receive higher amount of rainfall than areas on the leeward side. As such, mountain ranges and elevation affect the rainfall distribution resulting in rain shadow effect in the north-eastern parts of Choke-mountain and the ridges in the north-east of the basin. Moreover, a direct relation between rainfall occurrence and elevation is observed specifically for 17:00-18:00 LST. Further, results indicate that the rainfall distribution in the deeply incised and wide river gorge is affected with relatively low rainfall occurrence and low mean rainfall rates in the gorge areas. Seasonal mean rainfall depth is highest in the south-west area and central highlands of the basin while areas in the north, north-east and along the Blue Nile gorge receive the least amount of rainfall. Statistical results of this work show that the diurnal cycle of rainfall occurrence from TRMM estimates show significant correlation with the ground observations at 95% confidence level. In the UBN basin, the PR conditional mean rain rate estimates are closer to the ground observations than the TMI. Analysis on mean wet season rainfall amount indicates that PR generally underestimates and TMI overestimates the ground observed rainfall.

Effect of Bias Correction of Satellite-Rainfall Estimates on Runoff Simulations at the Source of the Upper Blue Nile

Results of numerous evaluation studies indicated that satellite-rainfall products are contaminated with significant systematic and random errors. Therefore, such products may require refinement and correction before being used for hydrologic applications. In the present study, we explore a rainfall-runoff modeling application using the Climate Prediction Center-MORPHing (CMORPH) satellite rainfall product. The study area is the Gilgel Abbay catchment situated at the source basin of the Upper Blue Nile basin in Ethiopia, Eastern Africa. Rain gauge networks in such area are typically sparse. We examine different bias correction schemes applied locally to the CMORPH product. These schemes vary in the degree to which spatial and temporal variability in the CMORPH bias fields are accounted for. Three schemes are tested: space and time-invariant, time-variant and spatially invariant, and space and time variant. Bias-corrected CMORPH products were used to calibrate and drive the Hydrologiska Byrans Vattenbalansavdelning (HBV) rainfall-runoff model. Applying the space and time-fixed bias correction scheme resulted in slight improvement of the CMORPH-driven runoff simulations, but in some instances caused deterioration. Accounting for temporal variation in the bias reduced the rainfall bias by up to 50%. Additional improvements were observed when both the spatial and temporal variability in the bias was accounted for. The rainfall bias was found to have a pronounced effect on model calibration. The calibrated model parameters changed significantly when using rainfall input from gauges alone, uncorrected, and bias-corrected CMORPH estimates. Changes of up to 81% were obtained for model parameters controlling the stream flow volume.

Hydrological Impacts of Urbanization of Two Catchments in Harare, Zimbabwe

By increased rural-urban migration in many African countries, the assessment of changes in catchment hydrologic responses due to urbanization is critical for water resource planning and management. This paper assesses hydrological impacts of urbanization on two medium-sized Zimbabwean catchments (Mukuvisi and Marimba) for which changes in land cover by urbanization were determined through Landsat Thematic Mapper (TM) images for the years 1986, 1994 and 2008. Impact assessments were done through hydrological modeling by a topographically driven rainfall-runoff model (TOPMODEL). A satellite remote sensing based ASTER 30 metre Digital Elevation Model (DEM) was used to compute the Topographic Index distribution, which is a key input to the model. Results of land cover classification indicated that urban areas increased by more than 600 % in the Mukuvisi catchment and by more than 200 % in the Marimba catchment between 1986 and 2008. Woodlands decreased by more than 40% with a greater decrease in Marimba than Mukuvisi catchment. Simulations using TOPMODEL in Marimba and Mukuvisi catchments indicated streamflow increases of 84.8 % and 73.6 %, respectively, from 1980 to 2010. These increases coincided with decreases in woodlands and increases in urban areas for the same period. The use of satellite remote sensing data to observe urbanization trends in semi-arid catchments and to represent catchment land surface characteristics proved to be effective for rainfall-runoff modeling. Findings of this study are of relevance for many African cities, which are experiencing rapid urbanization but often lack planning and design.

Inter-comparison of satellite rainfall products for representing rainfall diurnal cycle over the Nile basin

In this study, the authors inter-compared the performance of three satellite-rainfall products in representing the diurnal cycle of rain occurrence and rain rate over the Nile basin in eastern Africa. These products are the real time (RT) and post-real-time (PRT) (bias adjusted) versions of Tropical Rainfall Measuring Mission (TRMM) and other sources product known as TRMM-3B42 and the National Oceanographic and Atmospheric Administration Climate Prediction Center (NOAA-CPC) product which is based on the CPC morphing technique (CMORPH). The rainfall diurnal cycles are re-produced using these products with specific focus on assessing effects of geographic location and topographic features. The performance of the satellite products in representing rainfall diurnal cycle shows large variation over the Nile basin. The products overestimate rain occurrence over the lakes, islands, and shores and underestimate occurrence over mountain tops. Overall, CMORPH performs better than TRMM-3B42 RT and TRMM-3B42 PRT in capturing the diurnal cycle of rain rate in Lake Tana basin. However, the difference between the two products is very small for Lake Victoria basin, where both products perform more favorably. Over most of the Nile basin areas, the use of fine versus coarse temporal and spatial resolution of the CMORPH product showed large differences for diurnal cycle of rain occurrence than that of rain rate. Results also show that the bias adjustment of TRMM-3B42 product does not necessarily bring improvements probably since the adjustments are not performed based on local rain gauge data.

Attribution of changes in the water balance of a tropical catchment to land use change using the SWAT model

Changes in the water balance of the Samin catchment (277.9 km2) on Java, Indonesia, can be attributed to land use change using the Soil Water Assessment Tool (SWAT) model. A baseline-altered method was used in which the simulation period 1990 – 2013 was divided into four equal periods to represent baseline conditions (1990 – 1995) and altered land use conditions (1996 – 2001, 2002 – 2007, and 2008 – 2013). Land use maps for 1994, 2000, 2006 and 2013 were acquired from satellite images. A SWAT model was calibrated for the baseline period and applied to the altered periods with and without land use change. Incorporating land use change resulted in a Nash Sutcliffe Efficiency (NSE) of 0.7 compared to 0.6 when land use change is ignored. In addition, the model performance for simulations without land use change gradually decreased with time. Land use change appeared to be the important driver for changes in the water balance. The main land use changes during 1994 – 2013 are a decrease in forest area from 48.7% to 16.9%, an increase in agriculture area from 39.2% to 45.4% and an increase in settlement area from 9.8% to 34.3%. For the catchment, this resulted in an increase of the runoff coefficient from 35.7% to 44.6% and a decrease in the ratio of evapotranspiration to rainfall from 60% to 54.8%. More pronounced changes can be observed for the ratio of surface runoff to stream flow (increase from 26.6% to 37.5%) and the ratio of base flow to stream flow (decrease from 40% to 31.1%) whereas changes in the ratio of lateral flow to stream flow were minor (decrease from 33.4% to 31.4%). At sub-catchment level, the effect of land use changes on the water balance varied in different sub-catchments depending on the scale of changes in forest and settlement area.

Hydrological Balance of Lake Tana, Upper Blue Nile Basin, Ethiopia

In recent years, few studies are presented on the water balance of Lake Tana. In these studies, the water balance is closed by unknown runoff contributions from ungauged catchments. Studies relied on simple procedures of area comparison to estimate runoff from ungauged catchments. In this study, emphasis is on regionalisation approaches by the use of physical catchment characteristics and a regional model. For runoff modelling, the HVB-96 model is selected while automated calibration is applied as based on a Monte Carlo procedure. Closure of the lake water balance was established by comparing measured to estimated lake levels. Results of daily lake level simulation show a relative volume error of 2.17% and a Nash–Sutcliffe coefficient of 0.92. Results show runoff from ungauged catchments of 527 mm/year for the simulation period 1994–2003 while the closure term only is 85 mm. Compared to previous works this closure term is smallest.

Closing horizontal groundwater fluxes with pipe network analysis: An application of the REW approach to an aquifer

The Representative Elementary Watershed (REW) approach for modeling the hydrologic response of watersheds is based on the discretization of a catchment into hydrologically sensitive control volumes. Global balance laws for mass, momentum (and energy) are formulated for the volumes. An implementation of the approach requires closing unknown REW-scale mass fluxes and forces exchanged across the REW-internal control volume boundaries and between REWs. In the present paper we focus exclusively on the horizontal groundwater flow, while neglecting other hydrological processes such as surface runoff, subsurface stormflow and surface-groundwater interaction. Here we describe a simplified groundwater modeling approach, which is based on the parsimonious estimation of mass fluxes exchanged laterally between REWs. The procedure employs principles of mass and energy conservation as stated by the Kirchhoff laws. The problem is reduced to the solution of a coupled system of linear equations in terms of inter-REW groundwater fluxes and a dimensionless ratio @Q between the local saturated zone depths y^s and spatial length scales @L over which piezometric head gaps are dissipated. The equation system is solved by successive approximation, as suggested by the Cross method, which is used in engineering practice for resolving resistor networks under steady state conditions. The procedure converges rapidly for complex network configurations. It is shown that for given boundary fluxes imposed at the watershed edges, mass fluxes exchanged in-between REWs can be calculated in an unique and consistent fashion. The paper briefly reviews the essential governing equations, introduces the formulation of the problem in terms of the Kirchoff laws and shows the numerical solution of the network. Simulation results for an application to the Hesbaye groundwater system in Belgium are presented.