Southeastern Ethiopia
6.2. Materials and Methods
6.2.1. Spatial distribution of current (2005) and projected (in the 2020s, 2050s, and 2080s) total population of Weyib River basin
Spatial distribution (Wereda level) of current population of the Weyib River basin has been compiled from Genale-Dawa River Basin Integrated Resources Development Master Plan Study Final Report (GDMP, 2007). According to the suggestion made by NU (2004), annual population growth rate of 2.6% has been used (for the period 2006-2020) to project total population for different Wereda found in the basin in 2020. To project total population in 2050 (for the period of 2021-2050) 2.13% annual population growth rate has been used whereas to project total population in 2080 (2051-2080) 1.42% annual population growth rate was used. The spatial distribution of current and projected total population of Weyib River basin is presented in Table 6.1.
6.2.2. Total annual water demand estimates of Weyib River basin
Water demand estimation of the basin comprises water demand for (i) irrigation, (ii) domestic (drinking, sanitation, bathing and food preparation), (iii) commercial and public institutions (public schools, clinics, hospitals, offices, shops, bars, restaurants and hotels), (iv) industrial, (v) livestock, and (vi) water losses in the water supply system. Additional factors such as climatic variations and the level of service have been provided affect water demand have been taken into account.
The crop water requirement at each irrigation scheme (Bale Gadula, Tegona, and Tebel) found in the Weyib River basin has been calculated using CropWat 4.3 for Windows during Genal-Dawa River basin master plan development (GDMPD) study period (2000-2007). The two crops used for this calculation was wheat (131-day crop) in Bale Gadula and Tegona irrigation schemes and Maize (135-day crop) in Tebel irrigation scheme depend on the
subtracting crop water requirement from available rainfall for a given period. The total annual irrigation water demand of the three irrigation schemes of Weyib River basin is detailed in Table 6.2.
A standard guideline of 30-50 lpcd has been used as an urban domestic water demand in this study. Water demand of 30 and 50 lpcd were used to estimate the current (2005) and projected total domestic water demand, respectively. The water demand for commercial and public institutions usually linked directly to the population and is taken as 5% of the domestic water demand. These allowances have therefore applied to all Wereda (districts) in the basin.
Industrial water demand is not usually linked directly to the population. However, for the purpose of basin planning, it is assumed to use 10% of domestic water demand for all Wereda in the basin. To estimate livestock water demand, it has been taken a mean of 2 Livestock Unit (LU) per person for the whole basin. The average water demand for livestock has been taken as 25 litres per LU per day. In estimating water losses in the water supply system, a 20% of the totals of domestic, commercial and public institutions and industrial demands have been assumed.
Therefore, the average daily demand is taken to be the combined total of the domestic, commercial and public institutions, industrial, livestock demands and the system losses. The daily water demand in a Wereda varies depending on the time of day, the season and climatic conditions. Hence, the maximum daily demand has been taken as 1.15 times the average daily demand. The detail estimates for both current and projected water demand for each component (domestic, commercial and public institutions, industrial, livestock, system losses, average daily demand, maximum daily demand and total annual water demand) of the basin is presented in following boxes (Box A and B). The spatial distribution of current and projected total annual water demand of the Weyib River basin is shown in Table 6.3.
Box A: Year 2005
Domestic Water Demand (DWD) = 30 lpcd
= 0.030 m³/c/d CIWD = 5% DWD = 0.05 x 0.030 = 0.0015 m³/c/d IWD = 10% DWD = 0.10 x 0.030 = 0.003 m³/c/d LWD = 2 x 25 lpd = 50 lpcd = 0.053 m³ pcd System Losses = 20% (DWD + CIWD + IWD)
= 20% (0.030 + 0.0015 + 0.003) = 20 x 0.0345 = 0.0069 m³/d
Average Daily Demand (ADD) = 0.030 + 0.0015 + 0.003 + 0.050 + 0.0069 = 0.0914 m³/c/d
Maximum Daily Demand (MDD) = 1.15 ADD
= 1.15x0.0914 m³/c/d = 0.105 m³/c/d Total water demand per capita per day =0.105 m³/c/d Maximum Demand person per year = 0.105 x 365 m³
= 38.325 m³/ year Total water demand per capita per year =38 m³
Box B: Year 2020, 2050 and 2080 Domestic Water Demand (DWD) = 50 lpcd
= 0.050 m³/c/d CIWD ………. 5% DWD = 0.0025 m³/c/d
IWD .……….. 10% DWD = 0.0050 m³/c/d
LWD = 0.050 m³/c/d
SL = 20% (DWD + CIWD + IWD) = 0.0115 m³/c/d ADD = DWD + CIWD + IWD + LWD + SL
= 0.119 m³/c/d
MDD = 1.15 ADD = 1.15 x 0.119
= 0.13685 m³/c/d
Total water demand per capita per day= 0.13685 m³/c/d
Maximum Demand per person per year = 0.13685 x 365 m³/ year = 49.95 m³/ year
Total water demand per capita per year = 50 m³
Source: GDMP-II.3.1. Water Supply and Sanitation_Final Report
6.2.3.Water availability estimates of Weyib River basin
The standard procedure of ArcGIS interface ArcSWAT hydrologic model was applied to simulate spatial distribution of water availability in the basin for both current and future period under Ensembles of 3 ESMs for the RCP8.5, RCP4.5 and RCP2.6 scenarios. The theoretical description, hydrological component, data used and model set-up of ArcSWAT hydrologic model has been given in detail in chapter 4 of this study. The mean of 3 ESMs future downscaled daily maximum and minimum temperature and rainfall using SDSM have been applied into ArcSWAT model as an input to generate future water availability under various RCP scenarios considering that all other climatic and physiographic variables are supposed to be same in the upcoming period. The analysis of future water availability has then been carried out on an annual basis (Table 6.4 and 6.5) in the 2020s, 2050s and 2080s
time slices. Finally, the current (1984-2004) and future (in the 2020s, 2050s and 2080s time slice) water resources status of the basin is then calculated in Table 6.6 based on Water Stress Index (total available water divided by total number of population residing in the basin) as suggested by (IPCC, 2007) with the threshold value of 1000 m3 per capita per year below which a basin might be considered to be water stressed.