Water stress indices development

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to see their effect on the water balance of the watershed. The initial curve numbers of the newly developed land-uses were selected from the corresponding hydrologic conditions (see Appendix 10.1). Parameters calibrated for the existing land-use is assumed to have the same effect on water flows as in the land-use scenarios.

119 Water availability

Green water comprises most of the available water for the existing rainfed cultivation, forest cover and grazing lands in the watershed. The blue water, which is mainly the river discharge from the streams of the watershed, has not been accessible until now except for about 0.03% of the watershed that uses springs, river diversions, and hand-dug wells (see section 4 and Eguavoen et al. 2012). The inaccessibility is due to technological limitations and the transboundary water barriers to borrow the technology from elsewhere around the world.

FAO (1986) defines the green water (part of the rainfall that is available for plant growth) as effective rainfall (mm per unit time day, month or growing season;

Equation 7.2):

(7-2)

It is the part that is stored in the root zone after a rainfall event and that is ready for uptake by plant roots or stored as the soil moisture available for plant growth. The amount of the effective rainfall is affected by the climate and the soil factors like soil texture and structure, and the depth of the root zone.

The evaporation part represents water evaporated from stagnated water and bare soil after rainfall events. The evaporation cannot be avoided since it occurs at the pre-germination and early germination stage of cultivation activities (Rost et al. 2009).

It is difficult to separate this unproductive evaporation from the productive transpiration in SWAT as seen in the water balance accounting (see section 3.4). Rost et al. (2009) used a reduction factor of 0.85 to consider this unproductive rainfall component of the actual evapotranspiration. Therefore, actual evapotranspiration simulated using SWAT is used as available green water for the existing rainfed production considering the reduction factor for the unproductive evaporation. This factor is considered during categorization of the water stress index development. The 20% rule of a presumptive standard for environmental flow protection (Richter et al.

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2011) was used to establish the environmental flow requirement. This rule proposed that 80% of the natural runoff be allocated for environmental flow with 20% as available blue water.

Productive cultivated land, pasture and wood lands cover 99% and small-scale irrigation 0.1% of the watershed (Figure 7-2 and Table 7-1), i.e., almost the whole of the watershed is covered by productive land-use classes. The following water balance components are defined as available water options in this study based on the land-use scenarios developed (see section 7.4.3). Average actual evapotranspiration (AET) in 1992 to 2001 with factor of reduction as a green water, green water plus 20% river flow and green water plus all river discharge were used as different options of available water depending on the land-use scenarios.

Water demand

In this study, population data were used to quantify water demand for each water-scarcity land unit (WSLU) in the dry and wet seasons using data as given in Table 7-3.

WSLU were formed by overlaying HRU and population density data on the smallest administrative units (known as Kebele), which have an average size of 24 km². Rainfed agricultural activities starting from sowing to early harvesting occur in the period from June to October. These months were considered as wet season and the rest of the year as dry season. Most of the food and feed production of the year was in the wet season.

All agricultural water demands were distributed equally over these months. Domestic and economic (industrial) water needs were also distributed equally over all months of the year. Livestock drinking water demand was calculated from the water need for different cattle types in different seasons and distributed over both seasons according to the data. As observed during the field work in 2008 and 2009, the small rainfall events during the dry season are very important to supplement livestock feed. These rainfall events make the crop aftermath palatable and also make the grass- and bushlands green for a short period of time. 30% of the animal feed production needs were distributed equally over the dry months of the year.

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Table 7-3 Input data for basic water requirement calculation

Output Input Quantity Units Source

Domestic 18 m³/c/y Calculated

Drinking 5 l/c/d Gleick (1996)

Bathing 15 l/c/d Gleick (1996)

Food preparation 10 l/c/d Gleick (1996)

Sanitation 20 l/c/d Gleick (1996)

Population MoWRs database

Agricultural 1103 m³/c/y Calculated

Cereal production 401 m³/c/y Calculated

Energy requirement Kcal/c/d FAO (2004)

Cereal equivalent 4.04 Kcal/gm FAO (2003)

Water productivity 0.6 Kg/m³ Haileslassie et al. (2009b)

Livestock 698 m³/c/y Calculated

Drinking (dry/wet) (30/4) l/TLU/d Duguma et al. (2012b) Feed from grass 1557 m³/TLU/y Tulu et al. (2009_b) Population 0.64 TLU/c Haileslassie et al. (2009b)

Industrial^* 4 m³/c/y Calculated

Per cent of agricultural water need

1 % FAO (2013)

Total 2001 1125 m³/c/y Calculated

*Industrial water need in 2050 was assumed to be 10% of the agricultural water need. TLU stands for Tropical Livestock Units representing 250 kg life weight.

Of the animal feed, 30% was left as crop residue (Haileslassie et al. 2009b) and was not included in the livestock feed calculation, since it was already considered in the cereal production water demand. The remaining 70% was considered to be supplemented using grass production that needs 1557 m³ water per TLU per year (Tulu et al. 2009). Livestock population (TLU/c) was derived from human population and livestock per hectare data stated by Haileslassie et al. (2009b; Table 7-3). According to these data, the amount of the basic water requirement is 1125 m³/c/y, which is lower than the 1700 m³/c/y threshold value given in Falkenmark (1989). About 98 % (1103 m³/c/y) is attributed to agriculture where 62% is from livestock. The big water share for livestock indicates how livestock is important part of the system.

Figure 7-3 shows the population density (km^-2) of the Gumara watershed for 2001. The population of the country is estimated to increase from 65,891,874 in 2001 (World fact sheet 2001) to 174,800,000 in 2050 (Population Reference Bureau 2010).

This national population growth rate was applied on the study Kebele and town levels to compute the local population in 2050. The steep and fragile areas of the watershed were less populated as compared to the upstream and downstream plain areas. The average rural and town population densities are 266 and 4730 km^-2, respectively.

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These values are higher than the national (114), regional (122) and zonal (159) averages (CSA 2011).

Figure 7-3 Population density (per km²) of the Gumara watershed for 2001.

(Source: Ministry of Water Resources of Ethiopia)

Water stress indices

Although different water availability and water demand definitions are given in studies, categorization for defining the water stress level is fairly similar. The most frequently used categories used to identify the level of scarcity are 30%, 60% and 100%

of the available water. Smakhtin et al. (2005) categorized the following water stress indices (WSI) using long-term mean annual runoff and considering environmental flow.

1. WSI > 1: overexploited (current water use is tapping into EWR)-environmentally water scarce basins.

2. 0.6 ≤ WSI < 1: heavily exploited (0 to 40% of the utilizable water is still available in a basin before EWR are in conflict with other

uses)-environmentally water stressed basins.

3. 0.3 ≤ WSI < 0.6: moderately exploited (40% to 70% of the utilizable water is still available in a basin before EWR are in conflict with other uses).

4. WSI < 0.3: slightly exploited.

However, using these categories for rainfed agriculture and when all river discharge is diverted leads to wrong conclusions, since they were designed for blue water scarcity considering environmental flow. Considering the erratic nature of the rainfall in the area and the unproductive evaporation components in the available green water, a water demand exceeding 60% of the actual evapotranspiration is considered as highly scarce rainfed sub-watershed in this study.

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The different scenarios that were used to develop water stress indices are listed in Table 7-4. Two land-uses and three water availability status were used for the population of 2001 and 2050 to develop 8 water stress scenarios.

Table 7-4 Water stress indices scenarios

Scenario No. Code Scenario No. Code

1 LU1_ GN01 5 LU2_ GNEWR01

2 LU1_ GN50 6 LU2_ GNEWR50

3 LU2_ GN01 7 LU2_ GNYLD01

4 LU2_ GN50 8 LU2_ GNYLD50

LU1 is existing land-use practice, LU2 is land-use considering Gumara irrigation project (GIP). Water availability is GN (green water), GNYLD (green water plus water yield) and GNEWR (green water plus 20% of water yield that considers environmental water requirement-EWR). Total water needed was calculated based on the population in 2001 and 2050 indicated as 01 and 5o, respectively.