1. Basin Water Balance on the Central Rift Valley
Place
Dates
This project is funded by
The European Union

2. Table of Contents Water Balance Water Management Class Exercise
Central Rift Valley
Introduction
Lakes status
Environmental problems
Socio-Economic
Class Exercise
PET using Thornthwaite
Soultion

3. Water Balance Water Balance
Precipitation = Evapotranspiration + Water Surplus ± Water Storage variation
Hydrological Cycle.
Source: National Resources Conservation Service. "Conservation and the Water Cycle," 2012;

4. Water Balance Precipitation:
Phenomenon which transforms water moisture either into rain or into ice or snow
Evapotranspiration
Evaporation: physical phenomenon from which water passes from liquid to gas
Transpiration: biological phenomenon through which plants take water from the soil
Water Deficit
Difference btween potential evapotranspiration and reference evapotranspiration
Water Storage
Water stored in the soil that may be later be evapotranspirated
Water Surplus
Water that is not retained by the soil and becomes surface runoff or groundwater flow

5. Global Water Partnership, 2000
Water Management
Integrated Water Resources Management can be defined as a process which promotes the coordinated development and management of water, land and related resources, in order to maximize the resultant economic and social welfare in an equitable manner without compromising the sustainability of vital ecosystems.
Global Water Partnership, 2000

6. Central Rift Valley
It supports a wide variety of indigenous population of edible fish and other aquatic and wildlife
Its water resources are a focal point for large-scale development (irrigation, soda abstraction, fish farms, floriculture and horticulture activities, …)
Lake Ziway
Lake Langano
Lake Abiyata
Lake Shala
Major River Basins in Ethiopia
Source: Ministry of Water Resources
Central Rift Valley Basin
Source: Google Earth

7. Central Rift Valley Irrigation in greenhouses Open-air irrigation
2x106 m3/año
Open-air irrigation
x106 m3/año
Domestic consumption
7,3x106 m3/año
National Park
Industry
1x106 m3/año
Livestock
8x106 m3/año

8. Central Rift Valley: Lakes status
There has been significant research conducted to study various natural and anthropogenic factors and their environmental repercussions on:
Lake water level changes
Water Quality
Basic data of the Rift study lakes
Source: Ayenew, T. and Legesse, D. (2007)

9. Central Rift Valley: Environmental Problems
VISIBLE CAUSES
UNDERLYING CAUSES
Desforestation
Expansion of croplands
Lack of awareness
Construction materials
Lack of control
Coal production
Soil degradation
Poor water quality
Substances in the environment
Uncontrolled dumping
Livestock production
Irrigation with contaminated water
Excessive use of agrochemicals
Insecurity of land tenure
Poor investment
Inappropriate irrigation techniques
Precipitation and wind
Deforestation
Water availability decrease
Abusive water abstraction
Free use of water
Lake water level reduction
Biodiversity degradation
Ecosystemsdestruction

10. Central Rift Valley: Socio-Economic Aspects
Household income
Economic sectors on the basin
67% GDP on agrculture and livestock
10% GDP in industry
24% GDP in services
Agriculture
Smallholders: No irrigated land: 779,955 Ha – Irrigated land: 10,000 Ha
Large farms : Irrigated greenhouses: 500 Ha – Open-air irrigation crops: 750 Ha
Main source
Secondary source
Agricultural production
92.31%
3.98%
Livestock production
1.67%
71.20%
Forestry production
2.41%
3.15%
Petty trade
0.09%
6.02%
Others
3.52%
15.65%

11. Class Exercise: PET using Thornthwaite
Calculate the Monthly Thornthwaite Heat Index using the following formula:
𝑖= 𝑡 t = mean monthly temperature.
Annual Heat Index (I) is calculated as the sum of the Monthly Heat Indices:
𝐼= 𝑖=1 12 𝑖
Potential Evapotranspiration estimation should be done for each month, considering each month as 30 days long with 12 hours of theoretical sun per day, using the following equation:
𝑃𝐸𝑇 𝑛𝑜𝑛 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 =16· 10·𝑡 𝐼 𝛼
Being a
𝛼=675· 10 −9 · 𝐼 3 −771· 10 −7 · 𝐼 · 10 −5 ·𝐼
These values should then be corrected by the real length of the month and the theoretical sun hours for the latitude of the area, with the simple formula:
𝑃𝐸𝑇= 𝑃𝐸𝑇 𝑛𝑜𝑛 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 · 𝑁 12 · 𝑑 30 N = theoretical sun hours for each month
d = days of each month.

12. Class exercices: SolutionJ F M A S O N D
Annual
PET
59.0
60.9
79.3
76.2
81.7
72.0
67.5
70.1
70.5
72.9
64.5
56.5
831.3
RET
14.4
25.2
60.1
75.7
51.9
5.5
667.6
Shortfall
44.6
35.7
19.3
0.5
0.0
12.6
51.0
163.6 WR
0.8
24.0
80.0
43.6
Surplus
31.9
77.3
7.0
116.2