Reservoir operation analysis for Ribb reservoir in the Blue Nile basin

Chapter 16

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Getachew Welelaw Belay*, Mulugeta Azeze* and Assefa M. Melesse† *School of Civil & Water Resources Engineering, Bahir Dar University, Bahir Dar, Ethiopia, † Department of Earth and Environment, Florida International University, Miami, FL, United States

16.1 Introduction Water resources development and management remain at the heart of the struggle for sustainable development, growth, and poverty reduction. This has been the case in all industrialized countries, most of which invested heavily on both water infrastructure and institutions. Developing countries have not invested sufficiently in water infrastructure and institutions. In some developing countries such as Ethiopia, the unmet challenges of water resources management are beyond comprehension. Unless progress in water management is made, sustainable growth and poverty eradication can’t be achieved (Daniel, 2011). Ethiopia has 12 major river basins with an annual runoff volume of 122 billion m3 of water and an estimated 2.6–6.5 billion m3 of ground water potential. However, due to lack of water storage infrastructures and large spatial and temporal variations in rainfall, there is not enough water for most farmers to produce more than one time per year. The potential irrigable land is about 3.7 million hectares (Awulachew et al., 2007). Estimates of presently irrigated area vary, it ranges between 10% and 12% of the total potentially irrigable land that is currently under production using traditional and modern irrigation schemes (MoA, 2011). This demonstrates that water resources have yet made little contribution toward the development of irrigated agriculture. The exploitation of irrigation and hydropower potential has been recognized by the Ethiopian government as a key issue in the economic development of the country. To meet the strong need of food sustainability and increased energy demand, the Ethiopian government is trying to undertake a series of actions for the construction of power plants and irrigation projects, among these, Ribb Irrigation and Drainage Project is one of such efforts. Ribb reservoir is located in south Gonder zone of Amhara region, which is a multipurpose type of reservoir

Extreme Hydrology and Climate Variability. https://doi.org/10.1016/B978-0-12-815998-9.00016-6 © 2019 Elsevier Inc. All rights reserved.

for irrigation, domestic water supply, and flood protection. It is mainly intended for irrigating a total area of 19,925 ha. The irrigation area being serviced by a weir located downstream of the main dam and aimed to transform rain-fed subsistence agriculture into irrigated commercial agriculture. The other purpose is flood protection for communities living in the downstream lowland areas of Fogera and Kemkem plains (WWDSE and TAHAL Consulting Engineers Ltd, 2008). Implementation of such systems need good management and allocation of water and should be based on an insight in the evolution of past water use as well as an understanding of current demand and an awareness of possible future trends. Reservoir operation forms a very important part of the planning and management of water resources. But, it is a complex problem that involves many decision variables, multiple objectives as well as considerable risk and uncertainty (Oliveira and Loucks, 1997). In addition, the conflicting objectives lead to significant challenges for operators when making operational decisions. The release from a reservoir at any time, depends on the prevailing demands, the available water in the reservoir and projected inflow to the reservoir in the water year. Hence, the aim of this study is to configure and simulate the present and likely future water resource systems of Ribb River/Reservoir by employing HEC-ResSim model. The study developed operating guide curves. This rule curves define the release decision making according to the present reservoir storage. These are constructed from data in critical periods. Thus, it gives confidence to reservoir operators that the reservoir will have enough water to meet the future demand provided that the reservoir inflow is not less than the expected. Once the reservoir operators know the present state of the system, they can make a release decision according to the rule curves and their experience.

191

192 Extreme Hydrology and Climate Variability

16.2

3 1 Importance of reservoir operation plan discharge of 11.6 m s . The river flows in a westerly

The Ribb reservoir, water storage system is planned as a necessity for poverty alleviation and sustainable socioeconomic development purposes. This project has aimed at utilizing the water resources potential in the subbasin and transform rain-fed subsistence agriculture into irrigated commercial agriculture. The development of new infrastructures and facilities to meet this objective is costly. The hypothesis is that after investing such high cost for planning, designing, and construction purposes, most reservoir operations go through traditional and improper ways of decision making based on the subjective judgments by the operators. This exposes the system for huge losses before achieving planned goals. To address such problem in Ribb reservoir, undertaking reservoir operation analysis supported with simulation models is crucial. Developing reservoir operating guide rules which include the probability of water shortage and water spillage is important task to meet planned targets. This enables water managers to make decisions how releases and storage water over a period of time considering variable inflows and outflows/ demands. It also helps to optimize the performance of the system. The main objective of this study is to develop reservoir operation rule curves for the ongoing water resource development project of Ribb reservoir using HEC-ResSim model as follows: l

l

l l

estimate the total downstream water requirement for the project, assess the performance of the storage in irrigating the planned command areas, develop HEC-ResSim model for the Ribb subbasin, and establish reservoir operation guide curves to quantify an optimum release for the under development Ribb Irrigation and Drainage Project using HEC-ResSim model.

The hydraulics and hydrology system of the Ribb watershed and reservoir is presented to inform planners, decision makers, stakeholders, and concerned bodies on the operation and management of the reservoir. Alternatives are tested to develop an effective and efficient allocation of the reservoir water to the area.

16.3

Lake Tana and Ribb River

Ribb River watershed is situated mainly in Amhara National Regional state in South Gonder covering zone of Farta, Ebinat Woreda, and Debretabor town. It is located at a distance of 625 km northwest of Addis Ababa and 60 km from Bahir Dar City (Fig. 16.1). Geographical coordinate of the area is 12°350 N and 41°250 E and 13°540 N and 35°550 E. The river is about 130 km long and has a drainage area of about 1790 km2 and an average annual

direction and empties into Lake Tana. It is one of the main streams flowing into Lake Tana from the east. The Ribb River, with its tributaries, drains the western slope of the high mountainous area of Guna Terrara which rises to an elevation 4135 masl east of the town of Debre Tabor. In the low and middle reaches of the river is extensive alluvial plains bordering Lake Tana. The river meanders its way and flows slowly causing sediment deposits. It creates high water table and overflows the riverbanks during the rainy season due to insufficient riverbed conveyance. Consequently, major problems of flooding and water logging must be resolved in order to develop irrigated agriculture in the area (RIDP, 2010). The dam axis is located in between the geographic grid ref. UTM E 392174.64, N 1330225.76 and E 390813.45, N 1330018.02, at an altitude of 1880–1970 m. The left abutment is situated at an altitude of 1943 m, the center of the dam axis is situated at an altitude of 1873 m, and the right abutment is situated at an altitude of 1966 m. Access to the dam site is possible from the town of Addis Zemen using the existing dry weather road which is about 40-km long (RIDP, 2010). The dam is under construction as part of the Lake Tana Subbasin Four Dams Project. Water released from Ribb dam, at an elevation of approximately 1867 masl (at coordinates 12°020 3000 N and 37°590 4500 E) will be diverted to the irrigation sites by a weir for which four potential sites have been identified. The diversion weir will supply water to the left bank of the main canal and water to the right bank will be provided via a siphon. The Project command area is situated to the east of Lake Tana, on both sides of the river and on both sides of Addis Zemen-Wereta road. The Project area includes part or whole of 22 Kebeles (communities) with a total area of 473 km2. To the south and west, the area is bounded by a line, roughly following contour 1820 masl at Ribb River, and descending to contour 1795 masl near Wereta. The northern boundary follows contour 1820 masl. The total irrigable area on both banks of the river is 19,925 ha; (9104 ha on the rights and 10,821 ha on left banks of the river). Considering the soil, topographical characteristics and the existing land use, the estimated net irrigable area is 14,460 ha (RIDP, 2010).

16.3.1

Climate

The climate of Ribb Basin is marked by a rainy season from May to September, with monthly rainfall varying from 65 mm in May to 411 mm in July. Mean annual precipitation is about 1400 mm in the upper part and about 1200 mm in the lower part. The dry season from October to April has a total rainfall of about 8% of the mean annual rainfall (RIDP, 2010).

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

N

Ethiopia

Abay Basin

193

N

N

Ribb watershed

Legend High: 4517 Low: -236 Abay_Subbasins ws_polygon WaterBody Stream 0.10.05 0

0.1

0.2

0.3

0.4

Miles FIG. 16.1 DEM (m) and location map of Ribb River watershed, Tana subbasin.

Temperature variations throughout the year with average minimum t (19°C in December to 23°C in May), with minimum and maximum temperatures of 10.8°C and 30.1°C. Relative humidity varies throughout the year in Ribb dam site. The maximum value reaches up to 80% in the rainy months. Potential evapotranspiration values between 95 mm in December and 140 mm in April. Average daily sunshine hour is 7.1 with average wind speed of 1.3 m s1, and sunshine hours are low in the wet season. Annual isohyets over Tana subbasin show the highest rainfall over Gilegel Abay watershed is 1600 mm. By comparison, the mean annual precipitation of Gumara and Ribb (1300 mm) and Megech (1000 mm) watersheds is lower. The mean annual rainfall depth over Lake Tana is about 1000 mm, where Bahr Dar mean annual rainfall, at the Lake’s outlet is the highest rainfall site (RIDP, 2010).

16.3.2

Topography

The dam site is characterized by broad and flat flood plains, old bench forming terrace, and low to high relief basaltic hills with steep to moderately steep slopes. The right and left abutments of the dam are characterized by steeper slopes with slope angle of 35°–46°. There are developments of

relatively few shallow seated gullies at the reservoir catchment attributed to rill and gully erosion. The peak topography in the area is marked by Shikra hill, which is at an altitude of 1973 masl. The Upper Ribb watershed is characterized as a mountainous, wedge-shaped, and steepsloped (3.6%) watershed. The highest elevation of the watershed is about 4100 m in its southeastern part. The lowest topography land is at the dam site at an altitude of 1873 masl (Fig. 16.2).

16.3.3

Land use

The farming system in the watershed is mixed with dominantly oxen plowed cereal crop production and livestock rearing for centuries. The major land use types in the watershed include crop lands, grazing, very spares and patches of shrub/bushes, plantations, settlement, and miscellaneous lands. According to the Farta and Ebinat Wereda Agriculture and Rural Departments report, about 59% and 5.8% of the watershed area are used for annual and perennial crops cultivation, respectively. Grazing land occupies 11,241 ha that is about 16.4% of the total watershed area. In addition, 14,655 ha shrub lands, afro alpine and manmade plantations, which account for about 21.4% of the

194 Extreme Hydrology and Climate Variability

N

DEM 4109 1799

0

1.75 3.5

7

10.5

14

Miles

FIG. 16.2 DEM (m) of Ribb River watershed.

watershed area, and also used for grazing. Except a 158 ha of state owned natural forestland, natural and manmade forest area is a small fraction.

16.4 Ribb River basin hydrology and data availability The Ribb River, which is a major tributary of Lake Tana, originates at mainly Guna Mountain at the elevation of 4135 masl. It collects water from number of streams within the watershed area and its discharge varies greatly depending on the wet and dry seasons. The maximum discharge is 30.55, 16.98, 10.98, 3.52, 2.98, and 1.48 m3 s1 in August, July, September, October, June, and November, respectively, and the minimum flow is in March, 0.55 m3 s1. The major sources include rivers, streams, springs, and ground water sources. As related to flood discharge, the 1 in 5-year expected flood is 166 m3 s1; 1 in 10year is 205 m3 s1; 1 in 25-year is 253 m3 s1; 1 in 50-year is 287 m3 s1; and 1 in 100-year is 320 m3 s1 (RIDP, 2010).

16.4.1

was not possible to include daily rainfall data of more than 23 years due to lack of continuity of records. Monthly maximum and minimum temperature data records of 23 years (1992–2014) at Woreta and 19 years (1996–2014) at Addis Zemen stations were acquired for the analysis. Sunshine hours, relative humidity, and wind speed data were available only at the principal stations of Gonder and Bahir Dar. Therefore, monthly relative humidity, wind speed, and sunshine duration data are taken from Gonder station. Table 16.1 shows the selected rainfall stations.

16.4.2

Hydrological data

The computation of river discharge is the main task performed in the computation techniques of engineering hydrology. However, whether it is estimated indirectly from other hydrological variables or directly from river discharge measurement, the discharge data are only a sample in time of the behavior of the river. The hydrologist or the engineer

Climate data

Adequate quality data are the foundation for such type of study. Among the most important time series data needed for hydrological design and analysis of water resource projects are climatic data. Relevant climatic data include precipitation, maximum and minimum temperature, sunshine hours, wind speed, and relative humidity. There are many meteorological stations in and around the subbasin. For this study, five meteorological stations data were acquired from the National Meteorological Agency (NMA). These climate data are available with different length of records for each meteorological stations of Addis Zemen, Woreta, Gonder, Bahir Dar, and Debre Tabor. Rainfall data used in the analysis were 31 years (1984–2014) of monthly data at Woreta station. But, it

TABLE 16.1 List of selected meteorological stations in Ribb River sub-basin Station

Subbasin

Longitude

Latitude

1

Addis Zemen

Ribb

37.69

12.14

2

Woreta

Ribb

37.6

11.86

3

Gonder

–

37.41

12.51

4

Bahir Dar

–

37.39

11.59

5

Debre Tabor

Ribb

37.98

11.89

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

then must assess the worth of the data and its representativeness over the period for which the information is required, usually for the expected life of water resources structures. The Upper Ribb River flow data which is one of the inputs for HEC-ResSim model was obtained from the Ministry of Water Resources, Irrigation, and Energy (MoWIE) from 1982 to 2014. The time resolution of this flow data was daily and recorded at Upper Ribb flow gaging station.

16.4.3

Soil, land use, and crop data

The two major soil types, vertisols followed by fluvisols, are identified in the project area. The vertisols are the dominant soils in the area. These soils are deep and have clay content with shrink and swell property. The soils have slow internal drainage and difficult workability owing to the hard consistency. The fluvisols are soil units that occur mainly in the central part of the project area and are characterized by stratification of layers of different texture, the surface layer being predominantly clay loam to sandy clay loam. The soils are deep and generally well-drained RIDP (2010). The crop data (both existing and potential) include the main crops produced in the area, the planting and harvesting date of crops, and the cropping pattern. Other crop data such as crop coefficients, depletion level, and yield response of the crops are obtained. This crop data of the study area were also obtained from the Feasibility Study Report of Ribb Irrigation and Drainage Project (RIDP, 2010).

16.5 Data quality and missing data estimation 16.5.1

Missing climate data

Measured precipitation data are vital in hydrologic analysis and design. Since there are costs related to data collection, it is imperative to have complete records at every station. However, the actual condition in most of the data records is not complete for different reasons. For gages that require periodic observation, the failure of the observer to make the necessary visit to the gage may result in missing data. Vandalism of recorders is another problem that results in incomplete data records. Instrument failure because of mechanical or electrical malfunctioning can result in missing data. A number of methods have been proposed for estimating missing climatic data. But for this study, missing values of temperature, sunshine hour, relative humidity, and wind speed data were filled by using long-year average method. But to fill missing rainfall data at Addis Zemen and Woreta stations, it is found that the difference between the average annual precipitation at any of the adjacent stations is >10%. So, the regression method was used by developing the following formula for Addis Zemen and Wereta stations:

195

AZ ¼ 8:669 + 0:288G + 0:218BDr + 0:495DT

(16.1)

W ¼ 0:497  0:431G + 0:69BDr + 0:534DT

(16.2)

where AZ is Addis Zemen, W is Woreta, G is Gonder, BDr is Bahir Dar, and DT is Debere Tabor. After the acquiring relevant hydrological and meteorological data of the study area, it was important to check the quality of the data before further analysis and simulation was performed. This enables to identify if the data are homogenous, good quality, and complete with no missing data. Errors resulting from lack of appropriate data processing are serious because they lead to bias in the final results (Vedula and Mujumdar, 2005). In this study, data were passed through different data quality tests and appropriately filling of data gaps and adjusted for inconsistency and non-homogeneity using different techniques.

16.5.2 Stream flow data quality and missing data For this study, daily stream flow data are obtained from MoWIE from different gaging stations on various locations along Tana subbasin. Linear regression method was used to fill missing flow values of Upper Ribb station. A relationship between the available inflow data of Upper Ribb River and the two nearby rivers, Gumara and Abay is done by developing a formula using 6 years mean monthly flow of complete mean monthly flow data for each river. Then, missing values of the Upper Ribb gaging station were filled with data from the nearby station, Gumara primarily because its correlation coefficient is greater than Abay. The data set plotted with the inflow of Upper Ribb versus Gumara and Upper Ribb vs Abay was checked and linear regression line shows the goodness of fit, R2 values for each pair of data set. Fig. 16.3 shows the mean monthly flow patterns of the three rivers (Upper Ribb and Gumara in left Y-axis and Abay in right Y-axis) for the years 1983, 1984, 1987, 1988, 1996, and 2003. The collected data that are used for system simulation are stationary, consistent, and homogeneous. It should also be checked if the data contain an outlier. A simple but efficient procedure for screening these data is to test annual or monthly time series for the absence of trend and stability of variance and mean. This basic procedure is extended to include tests for the absence of persistence and relative homogeneity and consistency.

16.5.2.1 Test for relative homogeneity and consistency Homogeneity analysis is used to identify a change in the statistical properties of the time series. The causes can be either natural or man-made. These include alterations to land use and relocation of the observation station. Therefore, in order

196 Extreme Hydrology and Climate Variability

Upper Ribb and Gumara flow (m3/s)

160

3000

140

2500

120 2000

100

1500

80 60

Abay flow (m3/s)

FIG. 16.3 Flow patterns of Abay, Gumara, and Ribb Rivers.

1000

40 500

20

0 Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

Apr

Mar

Feb

Jan

0

Months Upper Ribb

Gumara

Abay

FIG. 16.4 Nondimensionalized rainfall from five stations and their average used for Ribb River watershed.

to select the representative meteorological station for the analysis of rainfall estimation, checking homogeneity of group of stations is essential. Homogeneity of the selected gaging stations monthly precipitation records was nondimensionalized using Eq. (16.3) (Linsley and Paulhus, 1983).
Pi Pi ¼  100% (16.3) P where Pi is nondimensional value of precipitation for the month i, Pi is average monthly precipitation for station i, and P is average yearly precipitation of the station.

The selected stations are also plotted for comparison. For illustration, Fig. 16.4 shows the result of homogeneity analysis plotted to check similarity between groups of stations. Same mode and pattern of the stations are observed and hence the group of stations selected is homogenous. If recorded data at a rain gage station have undergone a significant change during the period of record, inconsistency would arise in the rainfall data of that station. Checking for inconsistency of the record is done by the double-mass curve technique. This technique is based on the principle that when each recorded data come from the same population, they are consistent. The double mass curve

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

A. Zemen

Gonder

B. Dar

197

D. Tabor

Commulative rainfall of Woreta station (mm)

35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 0

5000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

Commulative annual average rainfall of group stations (mm) FIG. 16.5 Double mass curve of Ribb River subbasin for selected stations.

FIG. 16.6 Annual stream flow patterns of Ribb River (1982–2014).

technique is used to adjust precipitation records to take account of nonrepresentative factors such as change in location or exposure of rain gage. The accumulated totals of the gage in question are compared with the corresponding totals of group of nearby gages. If significant change in the regime of the curve is observed, it should be corrected. According to the double mass curves, all the stations were consistent (Fig. 16.5).

16.6 Ribb River flow characteristics The daily stream flow variations at the Upper Ribb gaging station are demonstrated by the 1998 wet rainy season (July–September) with annual flow of 633 Mm3 and by the 2002 dry year of 2002 seasonal flow of 70.39 Mm3,

nearly 11% of the wet year flow. Fig. 16.6 shows 33 years of historical time series data.

16.6.1

Reservoir inflow

The relevant hydrometric station to estimate river flows at the Ribb dam site is the Upper Ribb near Debre Tabor station (No. 1009). This station is operational since 1982. After missing flow data in the Upper Ribb station are filled using the linear regression method, the calculated flow values were used to generate flows at Ribb dam site. The most recommended guideline to transfer stream flow data to the point of interest is to use area ratio method described by Eq. (16.4). This method is often used when the ungagged site is on the same stream, upstream or downstream (Ries and Friesz, 2000).

198 Extreme Hydrology and Climate Variability

TABLE 16.2 Mean monthly flow of Ribb River at the dam site Unit

Q (m3 s21)

Mean monthly flow (Mm3)

Jan

0.61

1.65

Feb

0.43

1.05

Mar

0.48

1.27

Apr

0.53

1.38

May

1.13

3.03

Jun

3.43

8.89

Jul

19.61

52.52

Aug

33.9

90.81

Sept

15.42

39.96

Oct

5.03

13.46

Nov

2.04

5.29

Dec

1.11

2.97

16.7.2

222.29

Annual V (Mm3)

 Qs ¼ Qg

DAs DAg

n (16.4)

where DAs is drainage area at site of interest, DAg is drainage area of the gage site, Qg, Qs is discharge at gage and at site, respectively (m3 s1), n is a parameter that typically varies between 0.6 and 1.2. If the DAs is within 20% of the DAg (0.8  DAs/ DAg  1.2), then n ¼ 1. Daily flow records of 33 years hydrological period (1982–2014) were acquired and transferred to the dam site using the area-ratio method and used as an input data to HEC-ResSim model. Table 16.2 presents the transferred inflows at the dam site.

16.7 16.7.1

Irrigation and Drainage paper 56 (Allen et al., 1998), the conversion of ETo to evaporation of open water, with depth higher than 5 m, clear of turbidity, in temperate climate, varies between 0.65 and 1.25. For Ethiopia, the aridity correction factor is estimated to be 1.2 (RIDP, 2010). Mean monthly evaporation was estimated based on the Addis Zemen meteorological station (mean monthly climatic data of maximum and minimum temperatures) and Gonder meteorological station (mean monthly relative humidity, wind speed, and sunshine hours). The calculated evaporation was used for HEC-ResSim model as one of the inputs. The results are shown in Table 16.3. Reservoir seepage loss was estimated from Ribb Irrigation and Drainage Project feasibility study report as 6 Mm3 per year (RIDP, 2010).

Consumptive use and water demand Evaporation and evapotranspiration

In most instances, evaporation from open water is not directly measured but determined indirectly by one or more of several methods, such as water balance, energy budget, Penman Monteith formula, pan evaporation technique, and others. In this study, open water evaporation, ETo was calculated by using FAO CROPWAT version-8 program which uses the Penman-Monteith method and then applies an aridity correction factor. According to the FAO

Water demand

The Ribb Irrigation and Drainage Project is proposed to fulfill irrigation, domestic, and riparian needs. But, the riparian flow was not considered for this study. The project develops 19,925 ha of land and is planned for cultivation of cereals, oil seeds, vegetables, spices, pulses, commercial crops, and other horticultural crops (RIDP, 2010). The gross requirement of water for irrigation system is very much dependent on the overall efficiency of the irrigation system, which in turn is dependent on several factors: method of irrigation, type of canal, method of operations, and availability of structures. Here, 55% of overall efficiency is assumed based on the area ratio method of irrigation for the proposed areas, 14,459 ha surface irrigation and 5466 ha pressurized irrigation overall efficiencies are 50% and 70%, respectively. The crop coefficient, Kc, is basically the ratio of the crop ETc to the reference ETo. In this study, Kc values of the crops for the initial (Kc ini), mid-season (Kc mid), and end of the late-season stage (Kc end) were obtained mainly from Allen et al. (1998). Table 16.4 shows main crops, cropping patterns, and the percentage area coverage of each alternative. Calculated irrigation water requirement for each alternative was derived using the CropWat-8. Table 16.5 shows water requirement for Alternative-1 only. Alternative-1 considers 100% of irrigable command area of 19,925ha.

16.8

Ribb reservoir

Ribb reservoir is planned to develop a total irrigating area of 19,925 ha. The irrigation area being commanded by a weir located downstream of the main dam and aimed to transform rain-fed subsistence agriculture into irrigated commercial agriculture. The second purpose of the dam is flood protection for communities living in the downstream

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

199

TABLE 16.3 Climatic data and reference crop evapotranspiration (ETo) at station Woreta Month

Min temp (°C)

Max temp (°C)

Humidity (%)

Wind speed (m/s)

Sunshine (h)

Radiation (MJ/ m2/day)

ETo (mm/day)

Jan

11.2

28.7

37

1.3

9.1

20.2

4.01

Feb

11.1

27

31

1.5

9

21.5

4.41

Mar

12.9

30.1

32

1.5

7.4

20.5

4.68

Apr

13

29.6

35

1.5

7.8

21.6

4.86

May

13.5

30.1

44

1.6

7.3

20.5

4.75

Jun

13.3

26.5

67

1.6

4.1

15.4

3.48

Jul

13.1

25.2

79

1.1

3.8

15

3.02

Aug

13.2

24.6

78

1.1

4.9

16.9

3.26

Sep

12.8

25.2

69

1.1

6.4

19

3.61

Oct

12.7

27.1

56

1.1

7.7

19.9

3.87

Nov

11.2

27.1

48

1.1

8.6

19.8

3.71

Dec

10.8

28.5

40

1.2

9.1

19.6

3.74

TABLE 16.4 Cropping pattern of Ribb irrigation project % Area (Alt-2)

% Area (Alt-3)

Planting date

End of harvest

15

14

12

21 Nov–1 Dec

31 Mar–10 Apr

Sorghum

11

10

9

21 Nov–1 Dec

31 Mar–10 Apr

3

Chickpea

12

11

10

11 Nov–21 Nov

20 Apr–30 Apr

4

Pulses/field bean & lentil

7

6

6

21 Nov–1 Dec

31 Apr–10 May

5

Soybean

6

5

5

1 Dec–11 Dec

10 Apr–20 Apr

6

Sunflower

6

5

5

21 Nov–1 Dec

10 Apr–20 Apr

7

Peppers

10

9

8

21 Nov–11 Dec

31 Mar–20 Apr

8

Potatoes

5

5

4

21 Nov–21 Dec

21 Mar–20 Apr

9

Tomatoes

4

4

3

21 Nov–21 Dec

21 Mar–20 Apr

10

Cotton

7

6

5

11 Nov–21 Nov

10 May–20 May

11

Legumes/grain

6

5

4

1 Nov–11 Nov

30 May–9 Jun

12

Garlic

4

4

3

21 Nov–1 Dec

10 Apr–20 Apr

13

Onion

7

6

6

21 Non–1 Dec

10 Apr–20 Apr

100

90

80

No

Cropping pattern

1

Maize

2

Total percentage

% Area (Alt-1)

200 Extreme Hydrology and Climate Variability

TABLE 16.5 Alternative-1 monthly irrigation water requirement Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

NWIR (L s1 h1)

0.48

0.55

0.35

0.21

0.22

0

0

0

0

0

0.1

0.3

GIWR (L s1 h1)

0.87

1

0.64

0.38

0.4

0

0

0

0

0

0.1

0.5

Water demand (m3 s1)

17.4

19.9

12.7

7.61

7.97

0

0

0

0

0

2.5

9.1

FIG. 16.7 Status of Ribb dam viewed from downstream.

lowland areas of Fogera and Kemkem plains (WWDSE and TAHAL Consulting Engineers Ltd, 2008). Fig. 16.7 depicts construction progress of the dam. The Ribb Irrigation and Drainage Project is proposed to fulfill irrigation, domestic, and riparian needs. The project has planned for cultivation of cereals, oil seeds, vegetables and spices, pulses, industrial crops, and other horticultural crops. The crop water requirement was calculated based on the proposed percentage of land allocated for the different crops. The proposed cropping intensities (percent area coverage of the crops) to be grown annually, in wet and dry seasons, ranges from 160% to 190% (RIDP, 2010). For this study, the dry season irrigation agriculture is only considered.

16.8.1 Ribb reservoir physical characteristics data Ribb reservoir physical characteristics data were the most essential data for the study (Tables 16.6 and 16.7).

TABLE 16.6 Ribb reservoir dam characteristics No.

Feature

Dam

1

Watershed area

1592 km2 near Addis Zemen and 685 km2 at dam site

2

Mean annual flow

216.861 M

3

Dam type

Earth-Rock fill

Height

73 m

Crest length

800 m

Crest width

10 m

Crest level (top of Dam)

1945.5 masl

Bed level

1873 masl

4

Reservoir Max. length

6.27 km

Surface area

9.96 km2

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

TABLE 16.6 Ribb reservoir dam characteristics—cont’d No.

5

6

7

TABLE 16.7 Ribb reservoir pool characteristics—cont’d Outlet

Feature

Dam

Maximum water level

1945 masl

Normal reservoir level (NWL)

1940 masl

Dead storage level

1892 masl

Spillway

Elevation (masl)

Area (ha)

Total volume (m3)

Avg. elevation (masl)

Diameter 3m (40 m3 s21)

1898

212.1

19,527,000

1905.5

40

1900

236.1

24,007,000

1906.5

40

1901

248.9

26,432,000

1908.5

40

1902

259.9

28,976,000

1910.5

40

Max. spillway outflow

1060 m3 s1

1904

281.3

34,391,000

1912.5

40

Type

Uncontrolled

1906

300.9

40,215,000

1914.5

40

Crest level

1940 masl

1908

321

46,429,000

1916.5

40

Crest length

110 m

1910

342.1

53,061,000

1918.5

40

Width crest

20 m

1912

363.9

60,119,000

1920.5

40

388.3

67,640,000

1921.5

40

3 1

Peak discharge

2378 m s

1914

Max. head over the crest

5m

1916

413.5

75,655,000

1922.5

40

1918

443.8

84,217,000

1923.5

40

Outlet works

1920

474.3

93,393,000

1924.5

40

Capacity

40 m3 s1

1922

509.6

103,221,000

1925.5

40

Size of the conduit pipe

3 m diameter

1924

549.4

113,811,000

1926.5

40

1926

593.4

125,226,000

1927.5

40

Diversion weir

30 km d/s of the dam

1928

636

137,514,000

1929.5

40

1930

682.2

150,697,000

1931.5

40

1932

729.9

164,809,000

1933.5

40

1934

790.9

179,995,000

1935.5

40

1936

863.7

196,527,000

1936.5

40

1938

929.8

214,474,000

1938.5

40

1940

996.9

233,699,000

1939.5

40

TABLE 16.7 Ribb reservoir pool characteristics Outlet

Elevation (masl)

Area (ha)

Total volume (m3)

201

Avg. elevation (masl)

Diameter 3m (40 m3 s21)

1875

0.4

0

1888.5

15.4

1877

2.1

26,000

1889.5

26.6

1879

7

116,000

1890.5

34.4

1881

17.6

356,000

1891.5

40

1883

37.8

908,000

1893.5

40

1885

61.6

1,877,000

1895.5

40

1887

83.3

3,328,000

1897.5

40

1888

94.3

4,216,000

1899.5

40

1890

117.8

6,341,000

1901.5

40

1892

142.7

8,945,000

1902.5

40

1894

164.5

12,018,000

1903.5

40

1896

187.6

15,528,000

1904.5

40

1943

265,023,000

1945.5

287,509,000

Source: RIDP, 2010. Ribb Irrigation & Drainage Project (RIDP), Feasibility Study Report Volume-1 and 2. MoWR, Ethiopia.

These data include an elevation-storage-area relationship which describes the properties of the pool; while the crest elevation, length, outlets, and spillway information describe the dam. This data were directly taken from the prefeasibility report of Ribb Irrigation and Drainage Project. Spatial and temporal information about the entire system of Ribb River subbasin, including the location of the reservoir and the diversion, was obtained by delineating the watershed map using Arc-GIS software. This watershed map was one of the inputs of HEC-ResSim.

202 Extreme Hydrology and Climate Variability

16.9 Reservoir operation policy and modeling

reservoir and the release for demands at time step t, and Lt is the loss of water from the reservoir at time step t.

An operating plan or release policy is a set of guidelines for determining the quantities of water to store and to release or withdraw from a reservoir or system of several reservoirs under various conditions. Operating decisions involve allocation of storage capacity and water releases between multiple reservoirs, between project purposes, between water uses, and between time periods. Typically, a release plan includes a set of quantitative criteria within which significant flexibility exists for qualitative judgment. Operating plans provide guidance to reservoir management personnel. In modeling and analysis of a reservoir system, some mechanism for representing operating rules and decision criteria must be incorporated in the model. Reservoir system analysis models contain various mechanisms for making period-by-period release decisions within the framework of user-specified operating rules and criteria functions.

16.9.1

Reservoir/river system modeling

Goodmann and Major (1984) define systems analysis as “in a generic sense, systems analysis can refer to any orderly and scientific approach to problem solving. It includes traditional engineering methods and more recently developed mathematical methods in the field of operations research.” A large number of publications on applying systems analysis techniques to reservoir operation problems exist in the literature. Reservoir system analysis models can be categorized or classified in various ways. Popular operations research techniques include optimization methods, simulation, queuing theory, network flow theory, and game theory. Among these, optimization and simulation, prescriptive and descriptive, are extensively used in water resources problem. Simulation models are descriptive, and demonstrate what will happen if specified decisions are made. Optimization models are generally viewed as being prescriptive. In a simulation model, reservoir releases are determined by a set of predetermined operating rules. Through a series of simulations, these rules can be modified and improved until model results are judged acceptable. A reservoir system simulation model is based on a massbalance accounting procedure for tracking the movement of water through a reservoir-stream system by repeatedly solving the storage equation for a reservoir. In a general form, the mass balance or quantity equation for reservoirs is formulated as follows: St ¼ Sðt1Þ + It  Rt  Lt

(16.5)

where St and St1 are the reservoir storage at the end and at the beginning of time step t, It and Rt are the inflow into the

16.9.2 Reservoir operation simulation model (HEC-ResSim) HEC-ResSim 3.1 is used to simulate reservoir operations including all characteristics of a reservoir and channel routing decision support systems. The criteria for reservoir release decisions, an operation set, are drawn from a set of discrete zones and rules. The zones divide the reservoir by elevation and contain a set of rules that describe the goals and constraints that should be followed when the reservoir pool elevation is within the zone. In this model, reservoirs are a key focus point of the software. A reservoir is composed of a pool and a dam. HEC-ResSim makes the assumption that the pool is level and defined by an elevation-storage-area table. The dam element in the model of the reservoir forms an outlet hierarchy. The modeler can describe the different outlets of the reservoir in as much detail as is available or required. Both controlled and uncontrolled outlets are supported. An uncontrolled outlet, such as an overflow spillway, has no control structure to control flow.

16.10

Modeling and analysis

Water resources management is a complex topic that requires consideration of a broad range of social, economic, and environmental interests. As the world’s water resources become increasingly stressed, effective tools for management become more important. One tool often used in water resources management is decision support systems “an integrated, interactive computer system, consisting of analytical tools and information management capabilities, designed to aid decision makers” (Wakena, 2006).

16.10.1

Watershed setup

The purpose of the watershed setup module is to provide a common framework for watershed creation and definition among different modeling applications. This module is common to HEC-ResSim, HEC-HMS, and HEC-RAS. The watershed setup for Ribb River subbasin associated with a geographic region and area coverage has been configured. The watershed includes all of the streams, projects (i.e., reservoir and diversion), computation points, and hydrological and hydraulic data points for a specific area. All of these details together, once configured, form a watershed framework of the system. Maps are imported from external sources of Arc View GIS. Watershed setup of Ribb River subbasin and the reservoir system is shown in Fig. 16.8.

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

203

Rib Reservoir

FIG. 16.8 Watershed setup for Ribb River subbasin.

16.10.2

Reservoir network setup

After the watershed setup is completed, the reservoir network model is created and configured; the routing reaches and other network elements are added to complete the connectivity of the network scheme. Once the schematic is complete, physical and operational data for each network element are incorporated. Alternatives were also created that specify the reservoir network, operation set(s), initial conditions, and assignment of DSS pathnames (time-series mapping). The modeling elements that makeup the reservoir network for Ribb River subbasin include: junctions, reaches and reservoir, and computation points (Fig. 16.9). Each of these elements consists of one or more sub-elements. The following sections describe each element type beginning with the simplest elements, the junctions, and working up to the most complex, the reservoir. In Fig. 16.9, CP1 is control point which is the inlet point to the reservoir.

16.10.3

Reservoir physical components

The data that defines an individual reservoir element within the reservoir network consist of two conceptual types: physical and operational. Definition of physical parts is one of the most important parts in HEC-ResSim model. Even small changes affect the system behavior significantly and the impacts deteriorate or meliorate the result in the

simulation part. Input that should be considered for the physical part consists of the reservoir pool characteristics which are defined by the storage-elevation-area curve and the dam properties that consist of uncontrolled and controlled outlets.

16.10.3.1 Reservoir pool The property of the pool is described by elevation-storagearea curve. The main characteristics of the reservoir pool define the surface area and the volume of storage at the respective elevations. For Ribb reservoir, the input of elevation-storage-area is obtained in a spreadsheet format from feasibility study report (Fig. 16.10).

16.10.3.2 Dam, spillway, and diversion The crest elevation and crest length describe the dam. In HEC-ResSim, outlets are added to the dam to enable water to pass through to the downstream system. Diverted outlets are separate outlet groups to enable the reservoir to allocate flow through the diversion. Spillways are structures constructed to provide safe release of surplus water above the normal operation level of the dam to the downstream river stretches. A spillway can be a part of a concrete dam or connected to embankment dams. A diversion is a more complex element. It represents a “withdrawal” of water from the natural stream or reservoir for different purposes.

204 Extreme Hydrology and Climate Variability

Rib Reservoir CP1

FIG. 16.9 Reservoir network for Ribb River basin.

FIG. 16.10 Elevation-storage-area relationships.

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

16.10.4

Operation component

In a manner similar to the methods an operator may use, each reservoir in ResSim network must determine the quantity of water to release at each time step of a simulation run. To make this possible, an operation plan or scheme on release decisions is made. This operation plan is called an operation set ( Joan and Marilyn, 2013). An operation set consists of three basic features: zones, rules, and the identification of the guide curve.

16.10.5

System simulation setup

During the creation of the simulation model, it is a must to specify a simulation time window, a computation interval and the alternatives to be analyzed. The time widows given for present case were starting, lock back, and end time of simulation. Then, ResSim simulation creates a directory structure within the rss folder of the watershed that represents the “simulation.” Within this “simulation” tree will be a copy of the watershed, including only those files needed by the selected alternatives. Also created in the simulation is a DSS file called simulation.dss, which will ultimately contain all the DSS records that represent the input and output for the selected alternatives. In addition, elements can be edited and saved for subsequent simulations.

16.11 Reservoir system simulation and results After the watershed setup and reservoir network was completed, different alternatives with different decision rules were defined. The explicit system storage balance method was adopted to attain objective of the study. Hence, from the three modules of HEC-ResSim model, simulation module is the one where the simulation results are viewed with a number of trial and error iterations (Table 16.8).

TABLE 16.8 Simulation result of Ribb reservoir characteristics Location

Ribb reservoir

Parameter

Average

Maximum

Minimum

Storage (Mm3)

138.29

235.70

0

Elevation (masl)

1925.18

1940.2

1875

Controlled release (m3 s1)

6.95

45

0.01

Uncontrolled spill (m3 s1)

1.21

63.84

0

205

The simulation of the reservoir system considers a comparison among alternatives to obtain the maximum value of the objective function. For Ribb reservoir system, the purpose of the model is to determine the reservoir operation of releases over a given period of time with known stream flows at input points to the reservoir and other control points throughout the system for current condition. The flow data were taken from records of the available station at the main stream of Upper Ribb River and transferred to the dam site. In Ribb reservoir, three alternatives were defined based on 33 years of available inflow data, total water demands, the water losses through evaporation and seepage and the capacity of the reservoir. The total storage zone was divided into four zones: the Dam crest, flood control, conservation, and dead storage zone. These storage zones were leveled at different elevations. In this study, the conservation storage was a part of active storage which was guided by operation rules. The HEC-ResSim model was configured to make releases such that the level of the reservoir stays within the conservation zone. Based on the dead storage level (DSL) as a threshold value, the yearly inflows were classified as critical years with storage level that shows water deficit. Whereas, the normal years are the years that don’t show water stress or the storage of water is above the threshold value for all months. The results obtained from the simulation model showed that the reservoir could store water every year that can be used to fulfill irrigation and domestic water demands as shown in Fig. 16.11.

16.11.1 Alternatives and reservoir water balance performance indices A simple water balance was done on the system and the total useable volume of the reservoir was checked in balance of the total water requirement of the project for the defined three alternatives. The main reason to define the alternatives was to assess the water potential of the Ribb Irrigation and Drainage Project. This is because of the gap between the gross and the net irrigable land is large (i.e., 19,925 and 14,460 ha, respectively).

16.11.1.1 Alternative 1 In this alternative, the HEC-ResSim simulation was done by considering 100% of the irrigable command area fully developed. The analysis shows, from the 33 years historic inflow, 17 years were not satisfactory that did not fulfill the required water demand for the proposed area for all months. The performance of the reservoir to meet the target demand in the given 33 years is relatively low and can be seen in Table 16.9. Figs. 16.12 and 16.13 show the outputs obtained from the simulation model.

206 Extreme Hydrology and Climate Variability

Guide curve

Pool level

300,000,000

Stor (m3)

250,000,000 200,000,000 150,000,000 100,000,000 50,000,000 0 100

Flow (cms)

80 60 40 20 0 1985

1990

1995

2000

2005

2010

Rb Reservoir-Flood Control Opn_1......0 Stor.ZONE.1DAY

Rb Reservoir-Conservation Opn_1......0 Stor.ZONE.1DAY

Rb Reservoir-Inactive Opn_1......0 Stor.ZONE.1DAY

Rb Reservoir-Top Elevation Opn_1......0 Stor.ZONE.1DAY

Rb Reservoir-Pool Opn_1......0 Stor.1DAY

Time of Simulation

Rb Reservoir-Pool Opn_1......0 Flow.IN.1DAY

Rb Reservoir-Pool Opn_1......0 Flow.OUT.1DAY

Rb Reservoir Opn_1......0 Flow.MAXUM.1DAY

FIG. 16.11 Ribb reservoir storage and inflow-outflow rate (Alternative-1).

TABLE 16.9 Performance indices for Ribb reservoir Performance indices Reliability

Resilience

Vulnerability

Alternative

Rt (%)

Rv (%)

Res (%)

Vul2 (%)

1

86.1

76.2

30.1

10.4

2

92.2

86.2

31

8.4

3

94.4

91.4

32

7

Safe yield

100

100

100

0

16.11.1.3 Alternative 3 In this alternative, the simulation was done by considering 80% of the irrigable command area. The analysis shows that, out of the 33 years historic inflows, 6 years (18%) could not fulfill the required water demand for the proposed area. On the other hand, the amount of excess water that could be discharged through the spillway was higher than the two alternatives. The result for this alternative was interpreted as Ribb reservoir has a potential water resource to develop 80% of the gross irrigable command area (15,940 ha). The reservoir performance to meet the target demand was raised due to the decrease in irrigable area (Table 16.9).

16.11.2 16.11.1.2 Alternative 2 In this alternative, 90% of the irrigable command area was assumed. Here, out of the river inflows of 33 years, 9 years were critical years with minimum inflows that could not satisfy the required water demand for the proposed area. On the other hand, the amount of excess water that could be discharged through the spillway was higher than Alternative 1. The performance of the reservoir was relatively increased as the consequences of the decrease in irrigable area (90%). The result of this alternative could be expressed as, Ribb reservoir has a potential water resource to develop 90% of the gross irrigable command area (17,930 ha) (Table 16.9).

Safe yield

Safe yield is defined as that maintainable yield of water from reservoir water source which is available continuously during projected future conditions, including most drought years of record without failure. The calculated safe yield for Ribb reservoir was 146 Mm3 with 100% of reliability.

16.11.3 Performance indices of Ribb reservoir The water balance simulation for Ribb reservoir was done based on 33 years historic time series data at input point to the reservoir and the total water demand of the project. The next step was to assess the hydrological uncertainties based on the performance indices criteria and quantifying

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

70 60

Flow (cms)

50 40 30 20 10 0 1985

1990

1995

Ribb Reservoir_Outlet Opn_1......0 Flow.1DAY

2000

2010

2005

Ribb Reservoir.W.Supply Outlet Opn_1......0 Flow.1DAY

Ribb Reservoir.O.Spittway Opn_1......0 Flow.1DAY

FIG. 16.12 Ribb reservoir release rate (Alternative-1).

300 Volume (Mm3)

250 200 150 100 50 0 1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

Time (years) 300 Volume (Mm3)

250 200 150 100 50 0 1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

Time (years) 300

Demand

Volume (Mm3)

250 200 150 100 50 0 2004

2005

2006

2007

2008

2009 2010 2011 Time (years) FIG. 16.13 Ribb reservoir water balance (monthly available water vs water demand) for Alt-1.

2012

2013

2014

Supply

207

208 Extreme Hydrology and Climate Variability

the values of R-R-V under the standard operation policy of the reservoir. The simulation of Ribb reservoir for Alternative1 is depicted in Fig. 16.13.

TABLE 16.10 Ribb reservoir average monthly water balance (Mm3)

Month

Wet water balance (Mm3)

Normal water balance (Mm3)

Dry water balance (Mm3)

Sep

233.7

209.7

127.3

Oct

233.7

219.78

137.13

Nov

233.7

214.54

134.42

Dec

224.99

192.62

119.46

Jan

198.04

166.38

90.22

Feb

168.06

121.53

59.43

Mar

150.07

91.74

40.58

Apr

140.98

76.36

30.15

May

136.5

53.28

15.18

16.11.3.2 Resilience

Jun

158.81

55.17

21.76

Hashimoto (1982) defines resilience as a conditional probability of a year of success following a year of failure. It is also expressed as the inverse of the average failure duration. The resilience values obtained for all the defined alternatives of Ribb reservoir were 30.1%, 31%, and 32% for Alternative 1, 2, and 3, respectively (Table 16.9).

Jul

233.7

97.07

49.82

Aug

233.7

179.12

74.48

16.11.3.1 Reliability Reliability is essentially a measure of chance or assurance of an outcome from an operation when water is supplied to satisfy the requirements. The value of 100% reliability could be explained as the reservoir meeting the target demand for all the simulation periods. Time-based reliability (Rt) is the monthly time-based reliability of Ribb reservoir for the defined alternatives under full reservoir condition with values of 86.1%, 92.2%, and 94.4% for Alternative 1, Alternative 2, and Alternative 3, respectively. Volumetric reliability (Rv) is calculated based on the volume was 76.2%, 86.2%, and 91.4% for Alternative 1, Alternative 2, and Alternative 3, respectively (Table 16.9).

16.11.3.3 Vulnerability High values of dimensionless vulnerability reflect that the reservoir faces a shortage of flow to meet the demand in all simulation periods. For Ribb reservoir, the values are 10.4%, 8.4%, and 7.0% for Alternative 1, 2, and 3, respectively. Generally, reliability and resilience are both positive measures, the higher the better the performance, whereas vulnerability is a negative measure, the smaller the value the better the performance. So, the R-R-V values obtained for Ribb reservoir for the defined three alternatives were good. This indicates Ribb reservoir has a good potential to meet the target demand for the considered simulation periods. The estimated performance indices of Ribb reservoir for all defined alternatives at full reservoir condition are summarized in Table 16.9.

16.12

Reservoir rule curves

The maximum abstraction scenario was considered for the analysis of the Ribb reservoir operation. It is efficient use of limited water resources to release the water stored in the wet season in the reservoir according to the requirement of water demand by the crops and domestic water requirements during the dry season. The results of monthly inflows into the reservoir versus monthly water demand releases from the reservoir are displayed in Tables 16.10 and 16.11 and Figs. 16.11–16.13. From the figures, it can be

visualized that the total amount of runoff over the catchment of Ribb reservoir is equivalent to the water demand requirements of the project under the reservoir system. But due to climatic variability, there were variable annual inflows which lead to low system performance. The reservoir operation rule curves or guide curves are guide lines to manage the reserved water according to the available storage. These curves are developed based on the detailed sequential analysis for a combination of inflows and demands. The three guide curves are upper rule curve (URC), lower rule curve (LRC), and operating rule curve (ORC).

16.12.1

Upper rule curve

The main purpose of URC is to operate the expected available surplus storage during the flood seasons. The storage above this curve satisfies maximum water demands but, it tends to cause flooding. So, excess water above URC should be discharged downstream through the spillway and up to the maximum limit of bottom outlet.

16.12.2

Lower rule curve

The LRC is proposed to be a guideline for operation of the reservoir during minimum inflow. This curve determines the minimum level at which the requirements of the safe yield is satisfied. Water supply below the LRC is limited and reservoir water level is not allowed to fall below this curve.

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

209

TABLE 16.11 Estimated water releases of Ribb reservoir Irrigation demand (m3 s21) 3 21

Reservoir level (masl)

Alt-1

Alt-2

Alt-3

Max (m)

Operating (m)

Min (m)

0.15

0

0

0

1940

1937.5

1928

Oct

0.15

0

0

0

1940

1937.4

1927.9

Nov

0.15

2.54

2.28

2.03

1940

1937.2

1927.5

Dec

0.15

8.69

7.83

6.96

1939

1935.3

1925

Jan

0.15

17.39

15.65

13.91

1936.1

1932.2

1919.3

Feb

0.15

19.93

17.93

15.94

1932

1925.4

1911.8

Mar

0.17

12.32

11.09

9.85

1929

1920.6

1906.1

Apr

0.17

6.52

5.87

5.22

1928

1917.2

1902.4

May

0.15

9.78

8.8

7.83

1927

1911.1

1895.8

Jun

0.15

0

0

0

1931

1911.6

1899

Jul

0.15

0

0

0

1940

1921.8

1909

Aug

0.15

0

0

0

1940

1934.9

1920.2

Month

Domestic demand (m s

Sep

16.12.3

)

Operating rule curve

This curve is proposed to be a guideline for the operation of the reservoir at which the month-to-month requirements of all water demands are satisfied. In general, the storage between the URC and the LRC is the useful storage at which all water requirements are satisfied. The three guide curves were developed based on the reservoir water balance simulation of monthly inflow versus total water demand taking into consideration the water

FIG. 16.14 Operation guide curves for Ribb reservoir.

spillages and water stresses as criteria. The upper guide curve was developed based on the wet years that have water spillage; the operating guide curve was developed based on the normal water years where as the lower guide curve was developed based on the critical years that have water stresses. The data that were used to construct these curves are shown in Tables 16.10 and 16.11 and Fig. 16.14 shows the monthly water demands and the developed guide curves, respectively.

210 Extreme Hydrology and Climate Variability

The Ribb reservoir operation rule curves are guide lines to manage the stored water according to the present reservoir storage is presented in Fig. 16.14. These curves are LRC, ORC, and URC and the water levels (normal water level (NWL) and DSL).

16.13

Conclusions and recommendations

Ribb River is one of the main streams flowing into Lake Tana from the east with an estimated drainage area of about 1790 km2 and an estimated mean annual inflow of 460.6 Mm3. The Ribb River is one of the four main water resources around Lake Tana subbasin. It has a potential area of 19,925 ha for irrigated agriculture. Irrigation development aims to bring about increased agricultural production and consequently improves the economic, social, and environmental well-being of the community. It will contribute to raising the living standards of the country in general. The basin-scale water resources potential was assessed in relation to the expanding irrigation needs. An operating policy is developed to guide the operators at the reservoir site in decision making on water release at any time. Simulation was conducted by the selected reservoir operation model, HEC-ResSim based on 33 years historic stream flow data as an input to develop the operating guide curves. The water requirement of the irrigation project under the reservoir was calculated based on the proposed crop types and cropping pattern for the available command area using CROPWAT-8 software. The simulation of inflows versus water demands, useful storage, water spilling, and water stress was conducted for the defined three alternatives assuming that the reservoir is at full level initially. All the potential irrigable land which can be developed by the resources of Ribb reservoir was classified as 100%, 90%, and 80% to assess the water potential and to check the performance of the reservoir using the R-R-V criteria. Due to the annual climatic variability, there were variable annual inflows. Two major hydrologic incidences were observed during the study. The minimum inflows that cause water stress and maximum inflows that leads to water losses via spillage. The estimated amount of water losses discharged through the spillway during this study were 16.6, 47.9, and 60.4 Mm3 per year for Alternatives 1, 2, and 3, respectively. Based on the model results, Ribb reservoir system useful storage capacity for the defined three alternatives is sufficient to fulfill water requirements. The annual useful storage balance of Ribb reservoir is 224.75 Mm3, the annual inflows to the reservoir including rainfall on the reservoir is 237.12 Mm3, whereas the total water demand is 223.4, 203.4, and 183.3 Mm3 for Alternatives 1, 2, and 3. Losses were included in the alternatives. The riparian flow was estimated as 9 Mm3 per year, but not considered.

To manage the reservoir water appropriately, guide curves were developed based on the simulation results of HEC-ResSim model. Three rule curves are proposed for Ribb reservoir. The URC which is used to control the expected surplus storage, the LRC which determines the minimum level where only critical water requirements are satisfied and ORC, the level at which the day-to-day requirements of all water demands are satisfied. Therefore, from the analysis, it can be deduced that the available water source in the subbasin has sufficient capacity to satisfy the water requirements for all demands under the reservoir system for the proposed three alternatives even though Alternative 1 has shown relatively lower reservoir performance (reliability). The release guide rules can be used to operate the reservoir based on monthly water budget. This study recommends the following points to be included in the future studies for better water management and development in the watershed. 1. This work was conducted by employing HEC-ResSim 3.1 which does not have ability to simulate the rainfall runoff process in the catchment. As a result, outputs for reservoir simulation were done based on 33 years historical records of flow. Hence, in future studies, it is recommended to use stochastically generated stream flow instead of using historical records to develop better operating guideline that would fit with the real-time operation. 2. The simulation result shows the capacity of the proposed reservoir is unable to store high rainy season inflows. So, it is necessary to construct additional storage dam for two main purposes. First it protects the downstream areas from expected floods and second, it increases the probability of reservoir performance. 3. Research work should be done on the land use of the catchment. Flow monitoring to collect continuous good quality data and sedimentation assessment on the reservoir system are crucial.

References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper No. 56. Rome, Italy. Awulachew, S.B., Yilma, A.D., Loulseged, M., Loiskandl, W., Ayana, M., Alamirew, T., 2007. Water Resources and Irrigation Development in Ethiopia. International Water Management Institute, Colombo, Sri Lanka. Daniel, A., 2011. Water Use and Operation Analysis of Water Resource Systems in Omo Gibe River Basin. MSc Thesis, Addis Ababa University, Ethiopia. Goodmann, A.S., Major, D.C., 1984. Principles of Water Resource Planning. Prentice Hall Inc, Englewood Cliffs, NJ.

Reservoir operation analysis for Ribb reservoir in the Blue Nile basin Chapter 16

Hashimoto, T.J., 1982. Reliability, resiliency, and vulnerability criteria for water resource system performance evaluation. Water Resour. Res. 18 (1), 14–20. Joan, D.K., Marilyn, B.H., 2013. HEC-ResSim Reservoirs System Simulation User’s Manual Version 3.1. US Army Corps of Engineers, CA. Linsley, R.K., Paulhus, L.H., 1983. Hydrology for Engineers. McGrawHill, New York. MoA, 2011. Information Regarding Activities of Irrigation Development Department. Ministry of Agriculture, Addis Ababa, Ethiopia. Oliveira, R., Loucks, D., 1997. Operating rules for multireservoir systems. Water Resour. Res. 33 (4), 839–852. RIDP, 2010. Ribb Irrigation & Drainage Project (RIDP), Feasibility Study Report. vols. 1 and 2. MoWR, Ethiopia.

211

Ries, K.G., Friesz, P.J., 2000. Development of regression equations to estimate. Low durations and low-flow frequency statistics in New Hampshire streams. Water Resources Investigation Report 00-4135, USGS. Vedula, S., Mujumdar, P.P., 2005. Water Resources Systems: Modelling Techniques and Analysis. Tata McGraw-Hill, New Delhi, India. Wakena, T., 2006. Operational Analysis of the Cascaded Wadecha-Belbela Reservoir System in Ada’a-Liben Woreda (Oromia Regional State). Msc Thesis, Arbaminch University, Ethiopia. WWDSE & TAHAL Consulting Engineers Ltd, 2008. Lake Tana sub-basin four dams project, final feasibility report for Ribb irrigation project, Vol. III: hydrological report. Addis Ababa, Ethiopia.