An-Najah Staff

Extreme events, such as severe storms, floods, and droughts are the main features characterizing the hydrological system of a region. In the West Bank, Palestine, which is characterized as arid to semi-arid region; little work has been carried out concerning hydrological modeling. However some rainfall-runoff studies were done in the region using simple lumped models. The present research study deals with the hydrological modeling in Faria catchment under limited hydro-meteorological and spatial data. The goal has been to obtain reliable estimates of naturally available, surface water resources in arid to semi-arid environment of the West Bank, Palestine. For this purpose, an up to date physically-based and spatially distributed hydrological model (the newly coupled TRAIN-ZIN model) has been applied. Advances in geographical information system (GIS) made it possible to enhance the spatial model parameters estimation. Thus this study has been the first attempt of its kind in the Faria catchment which started from basic database development to adoptive model parameters identification and application. Understanding the processes of runoff generation is prerequisite to enhance the evaluation and quantification of water resources in the Faria catchment. Consequently such evaluation can be utilized in the development of best management practices that can be adopted to manage the scarce water resources in the catchment. The Faria catchment dominates the north eastern slopes of the West Bank and is a catchment of about 320 km2. It has arid to semi-arid characteristics with a Mediterranean climate which is characterized by hot and dry summers and mild and wet winters. The catchment receives a mean areal rainfall of 412 mm per annum falling during October to April. Rainfall events are highly variable in space and in time. Generally, they are of local extent with high intensities. Hence flood events of short duration occur mainly during winter months. Spring discharges are the direct contact between surface water and groundwater in the catchment. Wadi Faria is considered as a perennial Wadi since 11 springs provide the baseflow for the Wadi and preventing it from drying up during hot summers. The coupled TRAIN-ZIN model is used for runoff simulation. TRAIN simulates long term vertical fluxes between soils, vegetation and atmosphere whereas ZIN simulates short term runoff generation processes. The coupling layer of both models is the soil storage. During times of rain, ZIN model is active describing the filling of the soil storage and runoff generation by infiltration excess overland flow (IEOF) and/or saturation excess overland flow (SEOF) in time steps of minutes. During times of no-rain the soil module of TRAIN is active and calculates the emptying of the soil storage by evapotranspiration using the Penman-Monteith equation. Van-Genuchten method was integrated into the model to simulate water losses through deep percolation with a dynamical function. These calculations are important for modeling the next event, as they describe initial filling of the soil storage. With time steps of one day, TRAIN provides the missing long term simulation of soil moisture to ZIN. This modifies the ZIN model to a combined model that can be run on a continuous mode instead of single event oriented. A runoff generation map (terrain types) was developed for the Faria catchment with the help of aerial photographs. Based on this map, the model parameters for various terrain types are estimated. The terrain types represent the sub-units for the model’s runoff generation routine according to hydrologically relevant surface characteristics. For runoff concentration 1088 tributary catchments (model elements) were delineated whereas for channel routing the channel network was cut into 544 channel segments denoted by two nodes. Runoff delivery from model elements to the adjoining channel segment was timed by a uniform time lag. Small scale variability is not represented as a uniform time lag has been taken for all model elements. In the case of Faria catchment which was divided into 1088 sub-catchments with an average area of about 0.295 km2 scale variability can be neglected. Channel routing has been done using Muskingum-Cunge flow routing technique in which the Green-Ampt infiltration model was integrated to simulate the transmission losses. The parameter values for the three basic components in the coupled TRAIN-ZIN model, namely the runoff generation component, channel flow and transmission losses components and evapotranspiration component (climatic data for TRAIN part), were measured directly in the field (infiltration capacity and channel geometry), estimated from the literature (e.g. hydraulic conductivity, porosity, channel roughness, field capacity and others) and recorded (climatic parameters).Rainfall data from four tipping bucket rain-gauges and runoff data from two Parshall Flumes for three consecutive rainy seasons (2004-2007) were collected for the purpose of this study. The inverse distance weight (IDW) method was applied to estimate the spatial rainfall data from the pointwise raingauge stations in a five minute time step. Four considerable single rainfall events with different rainfall and runoff characteristics were used for model calibration and validation. The runoff simulation of event 1 with mainly SEOF and event 2 with more IEOF was used for model calibration. With the determined set of parameter values event 3 and event 4 were simulated for model verification. The traditional method of calibration which is based on a trial-and-error process was used. This method was carried out sequentially by adjusting the non measured model input parameters until the simulated values approximate the observed values. After the successful calibration and validation of the coupled TRAIN-ZIN model, continuous simulation of the entire rainy seasons 2004/05, 2005/06 and 2006/07 from October to April were achieved. This facilitated accurate assessments of seasonal water balances in the entire Faria catchment. Results of both events based and continuous simulations were optimistic to assume the applicability of the coupled TRAIN-ZIN model to the Faria catchment. The simulation results have been characterized by uncertainties of natural, data, parameter and model structure. In this study, parameters of the coupled TRAIN-ZIN model were determined through physical measurements carried out directly in the field (e.g. infiltration capacity) or from topographic maps (e.g. channels slope), from aerial photographs (e.g. channels geometry) or from information in the literature (e.g. hydraulic conductivity, porosity, Manning n, etc). Hence a certain amount of uncertainty is involved in parameter determination. These parameters usually may differ over years and even during various events of the same year. Therefore uncertainty assessment was carried out in this study to examine the sensitivity of the coupled TRAIN-ZIN model to the range of parameters uncertainty.  To evaluate how much and to what extend hydrological modeling can contribute to a quantitative analysis of the effects of land use and climate changes on catchment hydrology, the validated TRAIN-ZIN model was used to run land use and climate change scenarios to predict their effects on runoff characteristics as well as the overall availability of water resources in the Faria catchment. Consequently and in the face of the outstanding difficulties and challenges for managing the water resources in the Faria catchment, a set of proper management options were developed under the existing and future conditions.