Groundwater Recharge , Evapotranspiration and Surface Runoff Estimation Using WetSpass Modeling Method in Illala Catchment , Northern Ethiopia

Hydrometeorological information is important in planning and management of natural resources. The northern Ethiopia in general and Illala sub-basin in particular is lacking reliable information with regard to groundwater recharge, evapotranspiration and surface runoff. The main objective of the study is to estimate the distributed groundwater recharge, surface runoff and evapotranspiration amount of Illala sub basin using WetSpass modeling method. Long term mean hydrometeorological data and physical characteristics of the catchment such as land use/land cover, soil type, topography, groundwater level and slope are used as an input to the model. The mean annual groundwater recharge, evapotranspiration and runoff were found to be 66, 440 and 40mm respectively. Accordingly, recharge accounts for 12% of the precipitation while the rest 81% and 7% becomes evapotranspiration and surface runoff respectively. The study area is characterized by low groundwater recharge due to the presence of high evapotranspiration rate associated with high temperature, dry wind, low rainfall and relative humidity though it is a little bit large compared to some parts of the northern Ethiopia.


INTRODUCTION
Groundwater recharge is the process by which water percolates down the soil and reaches the water table either by natural or artificial methods to replenish the aquifer with water from the land surface.In arid and semi-arid areas, its assessment is a key challenge in determining sustainable yield of aquifers (Yongxin and Beekman, 2003;Crosbie et al., 2010).The same is true in many parts of Ethiopia like Tigray region where being semiarid it remains dry for major part of the year.
Hence, WetSpass was built as a physically based methodology for estimation of the long-term average, spatially varying, water balance components: surface runoff, actual evapotranspiration and groundwater recharge (Batelaan andDe Smedt, 2001, 2007).It is an acronym for water and energy transfer between soil, plants and atmosphere under quasi-steady state that was built upon the foundations of the time dependent spatially distributed water balance model (Batelaan andDe Smedt, 2001, 2007).
From a volume perspective, most water use in Illala catchment is appropriated from groundwater.Since the rainfall of the catchment is seasonal, most of the time, the tributaries and the main river are ephemeral.Hence, this forces the local communities to use subsurface water not only for drinking but also for domestic and in some cases for irrigation purpose.Though this groundwater is recharged from precipitation, utilization of this resource is going on without a basic understanding of the recharge amount and its areal distribution as well as the temporal and spatial variation of the other water balance components which creates a critical problem for its management in the catchment.Hence, estimation of rates of groundwater recharge in the area is crucial for sustainable utilization of the resource as well as its protection against pollution and depletion.In this paper groundwater recharge, surface runoff and evapotranspiration is estimated for the Illala catchment using WetSpass modeling method.

Land use/ land cover, climate and vegetation
Land-use/land-cover is a major controlling factor of watershed hydrology (Fetter, 2001).Landuse/land-cover of the area is dominated by cultivated land, bush land, forest, rural and urban settlement (Fig 3).The cultivated land which consists 64% of the land cover is distributed throughout the study area.Besides, it is the principal land-use of the catchment.Forest land accounts 4.1% which includes areas covered by eucalyptus trees, olea and acacia species.Bush land covers about 10.4% of the catchment which is dominated by acacia species.
Rural and urban settlement consists of 10.8% and 7.1%, respectively while the remaining 0.5%, 1% and 2.1% of the area are covered by water body, grazing and bare land respectively (Fig 3).

Geological and hydrogeological set up of the study area
The geology of Illala catchment is highly disturbed due to volcanic eruptions and dolor tic sills/dykes.Geology of the catchment is mainly dominated by limestone-shale-marl intercalation, limestone, dolerite and Quaternary sediments (Fig 4).Locally, the intensity of fracturing and faulting as well as the degree of weathering determines the aquifer potential and direct infiltration of the rock (Tesfaye and Gebretsadik, 1982).Hydrogeological conditions of Illala catchment is mainly controlled by the lithologies, geological structures and geomorphology.
Geological structures like faults and fractures play a principal role in groundwater movement and permeability of the catchment.

Limestone
In this unit, there are caves in the central part of the catchment along the riverbank.But towards both directions caves are reduced due to presence of shale and intrusion of dolerite as a result of low solution formation.Solution cavities, karstification, joints and travertine are also found in this unit at different parts of the catchment.Hence, in the catchment, the limestone formation has appreciable secondary porosity and permeability as result of fracturing and opening along and across the bedding planes.Wells drilled in this area yields up to 5 liters per second.In general, the limestone of the catchment can be taken as a moderately permeable aquifer (Fig 4).

Limestone -shale-marl intercalation
Mostly, this unit is dominated by limestone with intercalation of thin beddings of shale and marl.
There are various low discharges springs which are emerging at contact of the limestone and shale-marl beds with discharges not more than 0.5 l/sec.The permeability of this rock in northwestern and other parts of the catchment (Fig 4 ) shows a moderate value due to the presence of secondary porosity that developed as a result of fracturing and weathering.

Dolerite
This unit occurs as sills/dykes mainly in the limestone-shale-marl intercalation.Joints are common with 2-8 cm openings.Moreover, it shows fracturing and weathering.Hence, in some areas it facilitates infiltration until it reaches the massive part.This is evidenced by existence of high discharge springs in some parts of the study area, which reach about 1 l/s, emerging at the depressed areas of the dolerite unit.So, with increasing depth, the dolerite becomes massive.
Generally as figure 4 indicates, the dolerite unit of the study area is characterized with a very low to low permeability in the catchment though the fractured zone moderate degree of permeability.

Quaternary sediments
These sediments are formed due to erosion and depositional processes.However, erosion decrease currently due to different soil and water conservation practices conducted especially in some areas of the upper catchment.Since these sediments consist, proportions of different grain sizes that are transported from various parent materials; they become very important for infiltration and storage of groundwater.Their transmitting capacity and productivity vary from place to place depending on their grain size, sorting and thickness.Accordingly, these sediments range from low to high permeability (Fig 4).Gebretsadik, 1978;Tesfamichael, 2009;WWDSE, 2007;Girmay, 2011).

METHODOLOGY
Seventeen soil samples were collected based on soil color, landuse, landform, geology and field methods of soil testing using stratified sampling techniques.The sampling sites are distributed all over the catchment.They were collected by digging pits (subsurface profiles).The collected samples were analyzed in Tigray Agricultural Research Center laboratory.The land use and land cover map is produced using Landsat TM7satellite image taken in 2002 and field delineation by using GPS.
WetSpass modeling method was used to estimate groundwater recharge of the study area.As WetSpass modeling was developed for temperate regions by Batelaan andDeSmedt (2001, 2007), which has different climatic and land conditions compared to the tropics, some input parameters modification is used to apply it in tropical region.In temperate region, summer and winter have six month each while in Ethiopia summer contains four months and winter covers eight months.In addition to this, the seasons of rainfall period and land-use/land-cover are not the same.So, to apply the WetSpass for Illala catchment, input of the meteorological grid map was done using eight months of winter and four months of summer.Also, modified summer and winter land-use parameter tables were used.
Two types of inputs are required so as to run the WetSpass model: Parameter tables (dbf data) and grid map.ArcView (version 3.2) together with its spatial analyst extensions were used in order to prepare these input parameters.Inputs of landuse, soil and runoff characteristics parameter tables were required.Then, these tables were added to the maps as attribute.The runoff characteristics parameter tables contain runoff coefficient, slope and soil type for each corresponding landuse.In addition, since values in these tables are considered to be universal, no modifications are required for parameters of these tables.Topography, slope, landuse, soil, reference evapotranspiration and groundwater level maps were prepared.Hence, the topographic grid map, which is used to characterize the horizontal hydrological characteristics of the land surface and the slope data layer, describes the maximum change in elevations were derived from SRTM using global mapper software.Due to the lack of monthly data for groundwater depth, same map input were used for both seasons.Temperature, precipitation and wind speed parameters were prepared using the available meteorological data collected from national meteorological agency.

Hydro-meteorological data analysis
To estimate hydrologic balance for a given basin, each of the hydro-meteorological elements has to be quantified.Accordingly, rainfall, actual evapotranspiration, runoff and groundwater recharge have been estimated.

Rainfall
The rainfall pattern of the catchment is mono-modal type (Fig5) with 83% of the rainfall occurring in wet season (i.e.June-September).The rest 17 % occurs in the dry season from October to May.The spatial distribution of rainfall is shown in figure 6.

Potential evapotranspiration
Potential evapotranspiration can be determined in different methods.But so as to assure a well distributed contour grid map of potential evapotranspiration of the catchment, an equation developed by Seleshi (2002) that relates potential evapotranspiration and altitude with regression equation for Tekeze and Mereb basin was used (Table 1).Moreover, to check the reliability of this method to the area, PET is also calculated using the FAO Penman-Monteith method.
Potential evapotranspiration calculated using the two methods is given in figure 7. Therefore, as it is clearly seen in the figure 7, the reference potential evapotranspiration determined from the two methods are more or less similar.Table 1.Tekeze and Mereb basin monthly reference evapotranspiration (PET) and altitude relationships: Y = PET (mm/day), X = Altitude (m) (Seleshi, 2002).

Out puts of the WetSpass model
Annual groundwater recharge, annual actual evapotranspiration and annual surface runoff are the main outputs of the WetSpass model.A brief description of these outputs is given below:

Evapotranspiration
The annual evapotranspiration is calculated by WetSpass as a sum of evaporation from bare soil, transpiration of the vegetated cover, interception loss by vegetation and evaporations of open water body.Actual evapotranspiration is one components of water balance to determine groundwater recharge of Illala catchment using the WetSpass model.About 440 mm of water is lost through evapotranspiration from the catchment.This accounts for 81% of the catchment's annual precipitation.This shows that evapotranspiration is the main process by which water is lost in the catchment.This is attributed due to the high rate of radiation and the dry wind.About 79% of the total annual evapotranspiration is lost during summer season while the rest 21% is released in the winter season.This variation occurs due to difference in precipitation within the two seasons.

Surface runoff
To estimate the surface runoff of Illala catchment WetSpass uses runoff coefficient which varies its value with vegetation type, soil type and slope.The surface runoff of Illala catchment shows variation with land-use, soil type, slope, topography, precipitation and the other meteorological parameters (Fig 10).
The amount of surface runoff also shows variation in summer and winter season.Surface runoff in the Illala basin ranges from 0 to 191 mm with 40 and 39 mm of mean and standard deviation value respectively.The mean value represents 7% of the total annual precipitations of Illala.
From this, about 93% of the surface runoff occurs during the wet months (June to September) while the remaining 7% occurs during the dry months (October to May).This variation comes from rainfall difference in the two seasons.The rainfall exceeds the infiltration capacity of the soil during the wet season.This leads to high surface runoff.

Groundwater recharge
The WetSpass model estimates seasonal and annual long term spatial distribution amounts of groundwater recharge of Illala catchment by subtracting the seasonal and annual surface runoff and evapotranspiration from the seasonal and annual precipitation respectively.In general high values of groundwater recharge are observed in the bare-and cultivated land with sandy loam and loam soils.This is due to good permeability of these soils and gentle topography.
But all types of soils with forest, shrub and grazing land-use areas have resulted low amounts of groundwater recharge.This is caused due to the release of higher amounts of transpiration through the plant and grass leaf stomata respectively.From the simulated result, the safe yield of the basin can be assumed to be 25 percent of the total estimated recharge which is 17.2 mm/year.

CONCLUSION
The uneven distributions of the climatic parameters associated with variations of land-use/landcover, soil type, topography and slope are responsible for variations of the water balance element within the catchment.
The largest amount of evapotranspiration simulated for the catchment, relative to the groundwater recharge and the surface runoff, indicates that much effort is needed to change the environmental conditions of the catchment by applying some soil and water conservation practices.
By assuming 25% of the annual groundwater recharge can safely be abstracted, about 17.2 mm of the recharged water can be abstracted annually in a sustainable manner without having an adverse effect in the groundwater resource.
The result of WetSpass model can improve further if more seasonal groundwater depth data and new meteorological stations become available.Then, since the meteorological stations are located in the vicinity of Mekelle town with uneven distribution, additional meteorological station in the eastern part of the catchment around Araguren should be established.

Figure 8 .
Figure 8. Annual reference potential evapotranspiration map of Illala catchment.
According to the annually simulated surface runoff of the catchment(Fig 10), the north-eastern part of the catchment has highest surface runoff due to the presence of clay soil which has a low permeability that enhance for surface runoff.On the other hand, the western part of the catchment near Illala outlet, south western part, southern part around Kuha, south eastern part, northern part and some areas near Araguren has less surface runoff.This is caused due to sandy loam and loam soil types associated with forest and shrubs coverage of the area which hinder surface runoff.This shows these soil types have great impact on annual surface runoff of Illala catchment.use with sandy loam and loam soil yield the lowest surface runoff while rural settlement and agricultural land use types with clay soil yield the highest amounts of surface runoff in the catchment.Therefore, surface runoff is more governed by soil type when compared to landuse/land-cover type and slope in the study area.
catchment have less amounts of recharge due to low precipitation, impermeable soils and overcrowded settlements (Fig 11).
of groundwater can be safely abstracted annually without adversely affecting the groundwater resource of the catchment (Fig 12).The distributed safe yield estimation taking 25 % of the recharge estimated is shown on figure 12.