CSIRO PUBLISHING
www.publish.csiro.au/journals/trj
The Rangeland Journal, 2009, 31, 259–265
Impacts of micro-basin water harvesting structures in improving vegetative cover in degraded hillslope areas of north-east Ethiopia Sisay Demeku Derib A,C, Tewodros Assefa A, Belete Berhanu A and Gete Zeleke B A
Sirinka Agricultural Research Center, PO Box 74, Woldia, Ethiopia. Global Mountain Program (GMP), C/O ILRI, PO Box 5689, Addis Ababa, Ethiopia. C Corresponding author. Email:
[email protected] B
Abstract. Water is one of the most important entry points to improve rural livelihoods in drought affected areas of the north-eastern Amhara region in Ethiopia. Various attempts have been made to overcome this problem by making use of different water harvesting structures. However, the choice of structures has been difficult because of a lack of empirical evidence on the relative effectiveness of the different structures. An experiment was conducted from 2002 to 2004 to compare and evaluate three different water harvesting structures (eye-brow basin, half-moon and trench) against the normal seedling plantation practice by farmers (normal pit) as a control. Data on root collar diameter (RCD), diameter at breast height (DBH), height and survival rate of Acacia saligna tree seedlings was collected at 3-month intervals after planting and annual grass biomass production was also measured. Trench and eyebrow basin structures produced 68, 95, 52 and 44% increases in RCD, DBH, height and survival rate, respectively, 15 months after planting compared with the normal pit. Trench structures increased grass biomass by 41.1% compared with normal pits. Eye-brow basins are recommended on hillsides where stone is available while trenchs could be used where stone is scarce. The results indicated that well designed water harvesting micro-basin structures can mitigate the effect of dry spell shocks on tree seedling performance and land cover rehabilitation. They were also very effective in increasing grass biomass production indicating the potential for improving livestock feed on the available barren hillsides. Additional keywords: biomass production, seedling performance.
Introduction Recurrent droughts, erosion, flooding and drying of streams, springs and lakes appear to be increasing in Ethiopia although it is known as the ‘water tower of East Africa’ (Getachew 1999; Admasu 2003) with 12 major river basins (Awulachew et al. 2007) and high rainfall (mainly in the central, west and southwestern part of the country). The dry land area of north-eastern Amhara around the eastern escarpment is characterised by undulating topography with fertile bottom farmlands often located below hillslopes. It receives erratic rainfall (Meselech 2003) with dry spells in the main rainy season, and occasionally high rainfall intensity. It is also characterised by high population growth, which has forced people to clear remnant forest and trees from the hills and plow very steep slopes and marginal lands. This has disturbed the hydrological cycle by reducing infiltration, water holding capacity of the soil and percolation to the ground water table (Descheemaeker et al. 2009). This, combined with the erratic rainfall, results in high runoff rates from the hillsides affecting the fertile bottom lands through gravel deposition and formation and expansion of gullies. Millions of tree seedlings have been planted by different afforestation programs in order to increase the forest cover and biomass production and to improve the environmental conditions Australian Rangeland Society 2009
of the area. But in most cases, only 5–20% has survived (Thomas and LaVerle 1991). In addition to premature cutting by peasants and inadequate care (Thomas and LaVerle 1991), water stress is one of the major limiting factors, which reduces the survival rate and productivity of tree seedlings. One of the recent approaches to overcome the problems of low water availability in semi-arid areas, is the use of different water harvesting structures that fit with the existing social, technical and economical conditions of the society. The structures exist in different forms, depending on the catchments area and type of water storage mechanisms. Some of them are roof catchments, above-ground tanks, excavated cisterns, small dams and soil moisture (or in situ) water harvesting structures (Critchley and Siegert 1991; Mitiku and Sorsa 2002). The definition, history and classification of water harvesting techniques have been reviewed and discussed in various publications (e.g. Fattovich 1990; Critchley and Siegert 1991; Suleman et al. 1995). Excess runoff can be temporarily stored in micro-basin water harvesting structures around seedlings. These structures retain excess rainfall around the seedlings, and, thus, increase the time for infiltration and root absorption. In addition, they play an important role in supplying water directly to the root zone and also reduce sediment load and erosion at downstream sites. They are 10.1071/RJ09012
1036-9872/09/020259
The Rangeland Journal
S. D. Derib et al.
simple structures that can be dug by farmers with a good chance of replicability. The impacts of different types of micro-basin water harvesting structures have been investigated in different parts of the world. For instance, research results in Kenya showed that v-shape structures were effective (Mugwe et al. 2001). Similarly, experimental results on in situ water harvesting in eastern Ethiopia showed that those micro-basins of 100 m2 catchments area resulted in a 7.8% seedling survival increment for Acacia saligna seedlings compared with micro-basins with 50 m2 area (Abdelkdair and Richard 2005). Area exclosure with a cut-and-carry system is advised and practiced in the area to rehabilitate hillsides and protect valley bottom farmlands from gully erosion. The regional government is trying to distribute and certify degraded hillsides for individual farmers within the study area. However, the speed of regeneration of these closed hillsides, and survival rate of planted seedlings are very low because of the shallow soil depth, the very low vegetation cover and the technologies applied, i.e. mainly simple pit and area exclosure without application of water harvesting structures. In this region, there are many barren hillsides, a shortage of livestock feed and fuel wood, as well as the very serious damage that runoff causes to fertile lands at foot of the slope. However, there is a potential for improving the survival rate of tree seedlings, improving regeneration speed of closed areas and reducing the downstream negative effects of runoff by using appropriate water harvesting structures. Thus, the main aim of the study was to investigate the possible contribution of micro-basin water harvesting structures in improving survival rate of tree seedlings and to estimate their effects on biomass production. The specific objectives of this study were to: (1) evaluate the effect of different micro-basin water harvesting structures (eye-brow basin, half moon and trench) on growth and survival rates of tree seedlings and undergrowth in moisture stressed areas of north-eastern Amhara; and (2) formulate recommendations for development planners and experts, farmers and catchments treatment programmers on the use of micro-basin water harvesting structures.
Rainfall (mm)
1985–2005 2003 Mean daily temp. (°C)
Materials and methods Description of the study site The field experiment was conducted from 2002 to 2004 at Kalu ~350 km north of the capital city of Ethiopia, Addis Ababa at 108560 2500 N and 398460 5700 E. The area receives more than 1000 mm of average annual rainfall (Fig. 1). About half of the year is dry with a 50% probability of receiving a monthly rainfall of less than 14.7 mm, which is the threshold rainfall to fill interception losses before deep infiltration and runoff start (Sisay 2005). This contrast with the onset of the main rainy season is very rapid, with a mean rainfall in July of ~270 mm, when the vegetation cover is low because of the preceding long dry period (Fig. 1). This often leads to high runoff rates and severe soil erosion. November, December and January are the driest months of the year; and is also when extra moisture is required to increase seedlings survival. The rainfall received during the experimental period was below calculated as the average value from historical data (Fig. 1). Experimental set up The experiment was designed as a randomised complete block with four experimental treatments and three replications. Each treatment was established in a 50 25 m plot area with 2-m spacing between treatments (Fig. 2). Base line soil data were collected at the beginning of the experiment from each experimental plot (Table 1). It was very difficult to get uniform plots for all the replication on the hillslopes sides. Therefore, the blocking was aimed at minimising the between treatment error related to variations in the slope and soil depth. The treatments were half-moon, eye-brow basin, trench (Fig. 3), and normal pit as control. The construction techniques described by Carucci (2000) were employed for the study. All micro-basins had a planting pit and a water collection pit with different arrangements in space. The planting pit was similar for all treatments with 30 cm diameter and 50 cm depth. Half-moon and eye-brow basin structures had 2.5 m diameter semi-circular shape. The diameter of both structures lay along the contour line.
2002 2004 No. of years with monthly rainfall
