Precipitation Data from CMIP5-ESMs-RCPs Experiment: in Weyib River Basin, Southeastern Ethiopia
Chapter 4: Evaluation of the ArcSWAT Model in Simulating Catchment Hydrology: in Weyib River Basin, Southeastern Ethiopia
4.2. Materials and Methods
The Weyib River originates from the northern sides of the Bale Mountains and first flows north-eastwards then flow to East and south-eastwards for the remainder of its course.
Finally, it joins with Genale and Dawa Rivers near Ethiopia-Somalia border to strengthen its journey to Somali lowlands. The detail description of the study area has given in Chapter 3.
Figure 4.1 Study area: location map and weather stations of the study area
4.2.1. Materials and software program used
The materials employed in this study include Genale-Dawa River Basin Integrated Resources Development Master Plan Study Final Report from the Federal Democratic Republic of Ethiopia Ministry of Water Resources to extract some physicochemical properties of soil for different soil layers, soil and land use ArcGIS map layer that could be input to ArcSWAT hydrologic model.
The models/software programs used for this study are (i) Microsoft EXCEL to arrange meteorological and hydrological data as per the SDSM and ArcSWAT models data file format, develop various graphs, as well as to determine various statistical indices, for example R2, RMSE, SRS, NSE, and Pbias, (ii) NetCDF4Excel to manipulate the large-scale atmospheric variables from global archive into the study area, (iii) SDSM to downscale temperatures (maximum and minimum) and precipitation for baseline and future time period under different RCP scenarios, (iv) XLSTAT 2015 used to conduct Mann-Kendall trend tests
to verify whether there is statistically significant increasing/decreasing trend on future temperatures (maximum and minimum) and precipitation under different RCP scenarios for the study area or not, (v) Baseflow Program (Allen, 1999) to determine the fraction of streamflow that is contributed by base flow from observed/measured discharge data, (vi) Global Mapper 11 to manipulate the DEM data (meaning to merge, fill, and also configure different grid-size DEM into one to represent study area), (vii) Rainbow software to check homogeneity of long-term annual precipitation series at different meteorological stations since knowing an expected behavior of hydro-meteorological processes, mainly precipitation is an important for hydrological modeling, (viii) pcpSTST program (Stefan Liersch, 2003) to calculate daily parameters of precipitation data used by the weather generator of the SWAT model; for instance to calculate average total monthly precipitation from daily time series, standard deviation for daily precipitation in month, skew coefficient for daily precipitation in month, probability of a wet day following a dry day, probability of a wet day following a wet day, and average number of days of precipitation in month, (ix) ArcGIS to prepare spatial data (DEM, LULC and Soil maps) that can be used in ArcSWAT model, (x) and ArcSWAT model to simulate entire river basin and sub-basin scale current (baseline) and future (up to year 2100) surface and subsurface hydrological processes under climate change scenarios.
4.2.2. SWAT model description
SWAT is a public domain model actively supported by the USDA (United States Department of Agriculture) ARS (Agricultural Research Service) at the Grassland, Soil and Water Research Laboratory in Temple, Texas, USA. SWAT is a river basin scale, a continuous time, a spatially distributed model developed to predict the impact of land management practices on water, sediment and agricultural chemical yields in a large complex basin with varying soils, land use and management conditions over long periods of time (Neitsch et al., 2005). SWAT can analyze both small and large basins by subdividing the area into homogenous parts. As a physically based model, SWAT uses hydrologic response units (HRUs) to describe spatial heterogeneity regarding land cover, soil type and slope within a watershed. The SWAT system embedded within geographic information system (GIS) that can integrate various spatial environmental data including soil, land cover, climate, and topographic features. Currently, SWAT has embedded in ArcGIS interface called ArcSWAT.
SWAT is a physically based, continuous time (Lenhart et al., 2002) and computationally efficient hydrological model, which uses readily available inputs.
4.2.3. SWAT model approach
Basin can be subdivided into sub-basins and further divided into hydrologic response units (HRUs) to consider differences in soils, land use, crops, topography, and weather. The model has a weather generator that generates daily values of precipitation, temperatures, solar radiation, wind speed, and relative humidity from statistical parameters derived from mean monthly values. Depending on the data availability, SWAT model uses either SCS curve number method or the Green and Ampt infiltration method to estimate surface runoff. There are two approaches (variable storage coefficient method or the Muskingum routing) embedded in SWAT model to route flow through the channel. The model also has three methods (Penman-Monteith, Priestley-Taylor, and Hargreaves) to estimate potential evapotranspiration. The main equations used by the model are given below. SWAT splits hydrological simulations of a basin into two major phases: the land phase and the routing phase. The difference between the two lies in the fact that water storage and its influence on flow rates have considered in channelized flow (Neitsch et al., 2002). The detailed and complete descriptions have given in the SWAT technical documentation.
4.2.4. Hydrological components of SWAT model
Depending on the criterion given for hydrological model and taking into account the objective of the research SWAT model (distributed physically based) has been used for simulation of the hydrological processes of Weyib River basin. The SWAT model calibrated and validated to be applicable and efficient for simulation of streamflow in Bale Mountainous area of Shaya River basin (one of the tributaries of Weyib River basin) (Shawul et al., 2013).
The land phase which simulates the hydrology of the basin controls the amount of water, sediment, and nutrient and pesticide loadings to the main channel in each sub-basin whereas the routing phase monitors the movement of water, sediments, nutrients and organic chemicals through the channel network of the basin to the outlet.
In the land phase of hydrological cycle, SWAT simulates based on the water balance equation (Eq.4.1a) and hydrological components simulated includes, for instance, canopy storage, infiltration, redistribution, evapotranspiration, lateral subsurface flow, surface runoff, ponds, tributary channels and return flow (Arnold et al., 1998; Neitsch et al., 2005).
( )
∑
=
−
−
−
− +
= t
i
qw seep a surf day
t SW R Q E w Q
SW
1 0
(4.1a)
Where, SWt is the final soil water content, SWo is the initial soil water content on day, t is the time (days), Rdayis the amount of precipitation on day, Qsurfis the amount of surface runoff on day i, Ea is the amount of evapotranspiration on day i, Wseepis the amount of water entering the vadose zone from the soil profile on day i, and Qgw is the amount of return flow on day i.
All parameter are in mm.
Since sub-daily rainfall data was not available in the study area that can use for Green and Ampt infiltration method ('Green and Ampt, 1911'), Soil Conservation Service curve number equation (USDA, 1972) has been used to estimate the surface runoff of the catchment.
Penman-Monteith method (Monteith, 1965) which developed based on the temperature, solar radiation, and relative humidity and wind speed data records were used to estimate the PET.
The SCS curve number has described in Eq.4.2.
S) . (R
S) . Q (R
day day
surf 08
2
0 2
+
= − (4.2)
In which, Qsurf is the rainfall excess, Rday is the rainfall depth for the day, S is the retention parameter. All parameter are in mm. More detailed descriptions of the different model hydrologic components are listed in SWAT user’s manual (Neitsch et al., 2005). The retention parameter (mm) has defined in Eq.4.3.
S = 25.4 1000
CN − 10 4.3 The CN SCS is a function of the soil's permeability, land use and antecedent soil moisture conditions. SCS describes three antecedent moisture conditions (1) dry (wilting point), (2) average moisture, and (3) wet (field capacity). The moisture condition 1 curve number is the lowest value that the daily curve number can assume in dry conditions whereas the moisture condition 3 curve number is the highest value that the daily curve number can consider in wet conditions. The curve numbers for moisture conditions 1 and 3 have calculated from Eq. 4.4 and 4.5.
[ ]
(
100 2 exp202.533(100 0.0636) (100 2))
2 2
1 CN CN
CN CN
CN − + − ⋅ −
−
− ⋅
= (4.4)
( )
[
2]
2
3 CN exp0.00673 100 CN
CN = ⋅ ⋅ − (4.5)
In which, CN1 is the moisture condition 1 curve number, CN2 is the moisture condition 2 curve numbers, and CN3 is the moisture condition 3 curve numbers. Typical curve numbers for moisture condition 2 have listed in various tables (Neitsch et al., 2005) which are appropriate to slope less than 5%. However, in the Weyib River basin, there are areas with slopes greater than 5%. To adjust the curve number for higher slopes an equation developed (Williams, 1995) was used (Eq. 4.6).
[ ]
22 3
2 1 2 exp( 13.86 )
3 )
(CN CN slp CN
CN S = − ⋅ − ⋅ − ⋅ + (4.6)
In which CN2S is the moisture condition 2 curve number adjusted for slope, CN3 is the moisture condition 3 curve number for the default 5% slope, CN2 is the moisture condition 2 curve number for the default 5% slope, and slp is the average percent slope of the sub-basin.
The second phase of the SWAT hydrologic simulation, the routing phase, consists of the movement of water, sediment and other constituents (for instance, nutrients, pesticides) in the stream network. The rate and velocity of flow have calculated by using the Manning's equation. The main channels or reaches are assumed to have a trapezoidal shape by the model. The two kinematic wave model options (variable storage and Muskingum methods) are available to route the flow in the channel networks. The variable storage method uses a simple continuity equation in routing the storage volume, whereas the Muskingum routing method models the storage volume in a channel length as a combination of wedge and prism storages. For this study, therefore, the variable storage method (Williams, 1969) used in the ROTO (Routing Outputs to Outlets) (Arnold et al., 1995) model was applied as in Eq.4.1b.
Vstored = Vinflow − Voutflow 4.1b Where: Vstored is the change in volume of storage during the time step, Vinflow is the volume of inflow during the time step, and Voutflow is the amount of outflow during the time step. All parameters are in m3. Detail of the equation has given in SWAT manual.
4.2.5. SWAT model inputs A. Digital Elevation Model
A Digital Elevation Model (DEM) which describes the elevation of any point in a given area at a particular spatial resolution as a digital file. A topographic feature of study basin (slope
steepness, slope length, and defining of the stream network) with its characteristics (channel slope, length, and width) has been derived from the DEM. A 30 m resolution DEM obtained from ASTER official website was used for this study (http://gdem.ersdac.jspacesystems.or.jp/