Evaluation of groundwater resource potential by using water balance model: A case of Upper Gilgel Gibe Watershed, Ethiopia
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Abstract: Groundwater resource potential is the nation’s primary freshwater reserve and accounts for a large portion of potential future water supply. This study focused on quantifying the groundwater resource potential of the Upper Gilgel Gibe watershed using the water balance method. This study began by defining the project area’s boundary, reviewing previous works, and collecting valuable primary and secondary data. The analysis and interpretation of data were supported by the application of different software like ArcGIS 10.4.1. Soil water characteristics of SPAW (Soil-plant-air-water) computer model, base flow index (BFI+3.0), and the water balance model. Estimation of the areal depth of precipitation and actual evapotranspiration was carried out through the use of the isohyetal method and the water balance model and found to be 1 664.5 mm/a and 911.6 mm/a, respectively. A total water volume of 875 829 800 m3/a is estimated to recharge the aquifer system. The present annual groundwater abstraction is estimated as 10 150 000 m3/a. The estimated specific yield, exploitable groundwater reserve, and safe yield of the catchment are 5.9%, 520 557 000 m3/a, and 522 768 349 m3/a respectively. The total groundwater abstraction is much less than the recharge and the safe yield of the aquifer. The results show that there is a sufficient amount of groundwater in the study area, and the groundwater resources of the area are considered underdeveloped.
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Key words:
- Groundwater balance model /
- Groundwater resource potential /
- Recharge /
- Sustainable
注释: -
Table 1. Data collected, source and purpose
No. Data collected Sources of data Purpose 1 Long term meteorological data (1985-2017) National meteorological service agency (NMSA) of Ethiopia To determine aerial depth of precipitation and potential evapotranspiration 2 Hydrological data (1990-2013) Ministry of water, irrigation and energy office (MoWIE) For baseflow separation and to determine Runoff 3 Digital elevation model (DEM) 30 m × 30 m resolution ftp://ftp.glcf.umd.edu/glcf/Landsat/WRS2/ server To yield essential derivative products such as slope, flow accumulation and flow direction in the process of watershed delineation 4 Soil data Ministry of water, irrigation and energy office (MoWIE) and also food and agricultural organization (FAO) soil classification map was used in combination To determine available water capacity for different soil types and to determine actual evapotranspiration 5 Land use land cover data Ministry of water, irrigation and energy office (MoWIE) 6 Geology and hydrogeology 7 Existing borehole data Jimma zone water, mineral, and energy office, well completion reports of Jimma University and Jimma airport, from previous study around the watershed and Jimma zone water, sanitation and hygiene (WASH) report To locate existing boreholes in the watershed and to determine groundwater abstraction Table 2. Mean monthly aerial depth of precipitation by isohyetal method
1 2 3 4 5 No. Isohyets interval Mean isohyetal value, P1-2 (mm) The area between isohyets, A1-2 (km2) Col. 3 * Col. 4 1 1 460-1 480 1 470 4.41 6 479.5 2 1 480-1 500 1 490 5.67 8 453.5 3 1 500-1 520 1 510 7.79 11 763.0 4 1 520-1 540 1 530 14.73 22 531.2 5 1 540-1 560 1 550 33.29 51 597.8 6 1 560-1 580 1 570 60.56 95 078.1 7 1 580-1 600 1 590 90.92 144 558.6 8 1 600-1 620 1 610 206.30 332 137.0 9 1 620-1 640 1 630 222.33 362 394.6 10 1 640-1 660 1 650 349.81 577 190.4 11 1 660-1 680 1 670 1 231.83 2 057 152.1 12 1 680-1 700 1 690 313.83 530 367.9 13 1 700-1 720 1 710 152.26 260 371.2 14 1 720-1 740 1 730 92.11 159 358.8 15 1 740-1 760 1 750 53.23 93 146.3 16 1 760-1 780 1 770 36.02 63 758.3 17 1 780-1 800 1 790 26.25 46 988.9 18 1 800-1 820 1 810 20.40 36 928.8 19 1 820-1 840 1 830 18.37 33 610.1 Annual aerial depth of precipitation 1 664.5 mm/a Table 3. PET of the study area according to modified Penman method
Parameters Months Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. T (°C) 18.43 19.02 19.38 18.99 18.65 17.81 16.91 17.20 17.60 17.62 17.73 17.67 T (°F) 65.17 66.24 66.88 66.17 65.58 64.06 62.44 62.96 63.69 63.71 63.91 63.80 ea (mm/d) 15.9 16.4 16.9 16.4 16.1 15.3 14.4 14.7 15.1 15.1 15.2 15.1 RH (% ) 56.7 54.6 60.5 69.0 74.9 80.7 83.9 82.7 79.6 69.8 62.0 58.3 ed (mm/d) 9.01 8.95 10.23 11.32 12.06 12.35 12.09 12.16 12.03 10.54 9.42 8.81 U2 (m/s) 0.93 0.97 0.99 0.96 0.94 0.83 0.80 0.80 0.83 0.92 0.92 0.92 n (hrs./d) 7.14 7.22 6.91 6.34 6.51 5.59 3.84 4.21 5.52 6.85 7.51 7.67 N (hrs./d) 11.7 11.9 12.0 12.2 12.4 12.5 12.4 12.3 12.1 11.9 11.5 11.7 n/N 0.61 0.61 0.58 0.52 0.53 0.45 0.31 0.34 0.46 0.58 0.65 0.66 fa(n/N) 0.54 0.54 0.52 0.48 0.49 0.44 0.35 0.37 0.44 0.52 0.56 0.57 Ra (mm/d) 13.25 14.16 14.90 15.08 14.73 14.45 14.57 14.83 14.82 14.40 13.47 12.95 RI(1-r) (mm/d) 5.49 5.85 5.93 5.60 5.51 4.86 3.95 4.25 5.05 5.73 5.86 5.65 αTa4 (mm/d) 14.52 14.62 14.69 14.62 14.55 14.40 12.24 14.30 14.37 14.37 14.39 14.38 Ro (mm/d) 2.40 2.42 2.19 1.91 1.85 1.61 1.09 1.35 1.66 2.11 2.46 2.54 HT 3.09 3.43 3.74 3.69 3.66 3.26 2.86 2.90 3.40 3.62 3.40 3.11 ∆⁄γ 2.04 2.12 2.15 2.12 2.07 1.97 1.88 1.91 1.95 1.95 1.97 1.96 Eat 2.43 2.63 2.36 1.80 1.43 1.04 0.82 0.89 1.08 1.61 2.04 2.22 PET (mm/d) 2.87 3.17 3.30 3.08 2.93 2.51 2.15 2.21 2.61 2.94 2.94 2.81 PET (mm/month) 89.01 89.78 102.38 92.42 90.94 75.31 66.58 68.47 78.44 91.15 88.32 87.08 PET (mm/a) 1019.89 Table 4. Available water capacity of root zone and area coverage of soil types
Soil type Aerial coverage (km2) The area in (%) Chromic vertisols and dystric nitosols 1571 53.4 Dystric fluvisols 445 15.1 Eutric Fluvisols 700 23.8 Eutric nitosols 13 0.5 Orthic arcisols 212 7.2 Total 2941 100 Table 5. Adjusted WTRBLN for the whole study area
Parameters Months Annual (mm/a) Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. P 30.5 36.9 90.6 164.8 207.7 246.5 271.8 264.8 181.4 97.1 43.9 28.6 1664.5 DRO 1.5 1.8 4.5 8.2 10.4 12.3 13.6 13.2 9.1 4.9 2.2 1.4 83.2 Peff 29.0 35.1 86.1 156.6 197.3 234.2 258.2 251.6 172.3 92.2 41.7 27.1 1581.3 PET 89.0 89.8 102.4 92.4 90.9 75.3 66.6 68.5 78.4 91.2 88.3 87.1 1019.9 Peff - PET −60.0 −54.7 −16.3 64.2 106.4 158.9 191.6 183.1 93.9 1.1 −46.6 −59.9 561.4 AccPWL −166.6 −221.4 −237.6 - - - - - - - −46.6 −106.6 Sm 69.7 51.7 47.3 111.4 176.7 176.7 176.7 176.7 176.7 176.7 135.8 97.2 1573.0 $ \Delta $Sm −27.5 −18.1 −4.4 64.2 65.2 0.0 0.0 0.0 0.0 0.0 −40.8 −38.6 0.0 AET 56.4 53.1 90.5 92.4 90.9 75.3 66.6 68.5 78.4 91.2 82.5 65.8 911.6 SMD 32.6 36.6 11.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.8 21.3 108.2 S 0.0 0.0 0.0 0.0 41.1 158.9 191.6 183.1 93.9 1.1 0.0 0.0 669.6 TARO 0.0 0.0 0.0 0.0 41.1 179.4 281.3 323.7 255.7 129.0 64.5 32.2 1307.0 RO 0.0 0.0 0.0 0.0 20.6 89.7 140.7 161.9 127.9 64.5 32.2 16.1 653.5 DET 0.0 0.0 0.0 0.0 20.6 89.7 140.7 161.9 127.9 64.5 32.2 16.1 653.5 ROTL 1.5 1.8 4.5 8.2 31.0 102.0 154.2 175.1 136.9 69.3 34.4 17.5 736.7 Notes: P = Mean monthly aerial depth of precipitation, DRO = Direct runoff, Peff = Effective rainfall, PET = Potential evapotranspiration, AccPWL = Accumulated potential water loss, Sm = Soil moisture, $ \Delta $Sm = change in soil moisture, AET = Actual evapotranspiration, SMD = Soil moisture deficit, S = Surplus, TARO = Total available water for runoff, RO = Runoff without direct runoff, DET = Detention, and ROTL = Runoff including direct runoff. All values are in mm. Table 6. Mean monthly discharge of Gilgel Gibe near Assendabo river
Discharge Recording period (1990-2013) Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Annual (×106 m3) 24.57 18.56 19.31 25.58 54.96 117.1 223.7 311.3 267.2 159.6 77.38 39.12 1338.44 mm/a 8.35 6.31 6.57 8.70 18.69 39.80 76.07 105.8 90.87 54.27 26.31 13.30 455.10 Table 7. Estimated water balance components of the study area
Components Recharge (m3/a) Specific yield (%) Exploitable groundwater reserve (m3/a) Safe yield (m3/a) Abstraction (m3/a) Estimated values 875 829 800 5.9 520 557 000 522 768 349 10 154 684 Table 8. Summary of methods used and results obtained by different studies
Study area Method Results Reference Omo Gibe Baseflow separation Baseflow volume = 2 785 million m3 WAPCOS, (Moges, 2012) Omo Gibe Sub-surface drainage approach Annual groundwater recharge = 3 329 million m3 WAPCOS Omo Gibe Recharge area approach Annual replenishable recharge = 4 208 million m3 WAPCOS Omo Gibe 3D steady-state Finite Element Method based groundwater modeling code (TAGSAC) The groundwater resource potential = 4.38 billion m3 (Gedamu, 2015) Bulbul sub-basin Water balance method The groundwater recharge = 178 067 792 m3 (Shimelis, Megerssa and Fantahun, 2014) -
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