Preliminary Study on the Impact of Water Quality and Irrigation Practices on Soil Salinity and Crop Production , Gergera Watershed , Atsbi-Wonberta , Tigray , Northern Ethiopia

Possible long term effects on soil salinity and crop production due to the quality of water and irrigation practices is assessed in an area in Gergera Watershed in Atsbi-Wonberta, Tigray, northern Ethiopia. Ten water samples collected from hand-dug wells and small household ponds, and thirty soil samples from different depths up to ~100cm were tested for various parameters such as TDS, pH, anions and cations. Data indicate that both water and soil in terms of quality are acceptable for irrigation purposes. However, at present the soil salinity is not a serious issue but the data suggests its possible increase with time as indicated by two samples. Some of the issues such as use of sprinklers, organic manure, blending, seasonal crops are discussed in the light of maintaining the required quality, proper utilization of soil and water resources, and for sustainable development.


INTRODUCTION
Groundwater use for irrigation, domestic and other purposes is increasing with increasing population globally and related food insecurity problems.In Africa, increasing agricultural productivity is a key to poverty reduction.
The average rate of irrigation development for the Sub-Saharan Africa region (40 countries) for the last 12 years was about 43,600 ha/year, which is an average of 1090 ha/year for each country.Some counties like Tanzania, Nigeria, Niger, Zimbabwe and South Africa have an average rate of development over 2000 ha/year (FAO, 2001).
According to Ethiopian Ministry of Water Resources (2001), Ethiopia is endowed with a huge potential of water resources, with 122 billon m 3 annual surfaces runoff and 2.9 billion m 3 of groundwater.However, the county's water resource has contributed little to the country's socioeconomic development.Like most African countries, Ethiopian economy is The strategy aims at increasing agricultural productivity and production, and improving the rural living standards, which in turn would increase the demand for goods and services and will lead to industrial development.This could be attained partly by promoting irrigation (Mekuria, 2003).
As part of this strategy, the regional government as well as non-governmental organizations is engaged in water resource development activities both at household and community level to be used as a source of water for supplementary or complementary irrigation.
Currently, in Tigray, surface irrigation is the most predominant form of irrigation; it includes spring development, river diversion, flood spreading, micro-dams and pond systems.
Groundwater is also developed in different parts of the region as source of water for irrigation.
In the study area, with the introduction of water harvesting practices, groundwater and pond water utilization for irrigation by individual farmers has increased significantly.
As we know that availability of water by itself is not a guarantee for sustainable agricultural development, but its acceptability for different purposes like irrigation and domestic use is very important.Irrigation water quality problems may be caused by total mineral salts accumulation so that crops no longer produce well due to the development of sodic soils and accumulation of toxic levels of elements such as chloride, sodium and boron, these elements could make the land unproductive that incurs additional cost soil and water for leaching.At the same time nature of the soil and its quality with time is also important to sustain the required results.All soils contain some amount of soluble salts.Many of these salts act as sources of essential nutrients for healthy growth of plants.However, when the quantity of salts in the soil exceeds a particular value, growth, yield, and quality of most crops is adversely affected to a degree depending upon the kind and amount of salts present.Thus, over a period of time, salts that were previously distributed in the whole profile may selectively accumulate on the surface and give rise to saline soil.These are direct sources of salts on good quality lands.A good example is the use of high sodium-rich waters, which may lead to poor permeability and dispersion of soil.Salinity being one of the common problems in irrigation which can be built up with time though the rates varies on the salt content of the water used and can cause serious problems to soil quality and productivity.So, the issue of quality of water and soil is to be considered in the early stages of irrigated agricultural development programs and also is important in understanding and taking the correct management measures for long-term production.
With this background, the present paper tries to assess the quality of water (hand dug well and pond water) and soil; impact of water quality and irrigation practices on soil quality in terms of salinity; and suggests some techniques suitable for proper utilization of groundwater and surface water for irrigation both at household and community level in the Gergera watershed, Tigray.
Altitude wise, Atsbi Womberta district ranges from 1500-2800 m.a.s.l.The average daily temperature of the area is between 15°C and 30°C.The mean annual rainfall of the area is about 529 mm.The study area is drought prone with erratic, unevenly distributed rainfall and high run-off.The average rainfall in the area exceeds the potential evapotranspiration only in two months (July and August) of the year.Availability of adequate moisture is critical, in June at the start of the rainy season for germination; and in September for flowering, fruiting, crop production and tree growth.
The problem of land degradation in the area was high prior to 1997 due to energetic raindrop splash and high runoff.To overcome the problem of the resource degradation problem in the watershed, the District Bureau of Agriculture and Natural Resources has designed a watershed development program.With the full participation of the community, development strategies were mapped out and identified intervention measures to mitigate the consequences of land degradation, to rehabilitate degraded area and to increase land productivity.This resulted in the introduction of massive soil and water conservation structures like hillside terraces, trench bunds and re-forestation programs in the watershed area.Thus the area is now rehabilitated and is covered by increased biodiversity.This has not only helped to enhance the water resources in the area by raising groundwater table significantly but also has become a positive influential factor in irrigation development system.
Land use and land cover in the Gergera watershed is categorized as cultivated land, grassland, homestead and forest land.Major vegetable crops grown in the area are potatoes, hot pepper, onion, swiss chard and tomato.Considering the macro relief of Gergera watershed, physiographic nature ranges from almost flat to rugged mountain ranges.The corresponding slope range varies from 2% up to 80%.The land use of the area in relation to slope is given in table 1.The area of study is drained by a number of intermittent streams that flow from east to west direction and the drainage pattern of the study area is dendrite type (Fig. 1).In recent years, erratic and unreliable rainfall combined with the uneven topography, and the traditional agricultural practices, collectively are affecting the overall crop productivity in the area.Due to this the area faced food shortage even during the good rainy season and gradual decline in crop production and food in-security.However, the vegetation cover in the area has been improved with artificial plantation and area enclosure methods adopted with cut and carry system.

Farming System
The people of Gergera watershed exercise rain-fed, subsistence oriented mixed crop-livestock production farming system.The major crops and vegetables grown in the area are teff, barley, maize, wheat, bean, potatoes, hot pepper, onions, swisschards, lettuce and tomatoes.Recently, farmers are introducing fruit trees like Avocado, Guava, Banana and Papaya.The farmers of the study area use irrigation agriculture mainly using hand-dug wells, household farm ponds.
To maximize yield, farmers use artificial fertilizer especially urea and di-amonium phosphates.
These may contribute towards increase in the saline content or salinization process directly or indirectly.In the Gergera watershed there are about 500 households (above 80 thousand populations as per 1994 census) mostly involved in irrigation activity but with small land holdings.

Geology
The

METHODOLOGY
Gergera watershed boundary was delineated using topographic map.A reconnaissance survey was carried out in the watershed to record the total number of shallow hand-dug wells and household ponds present and that are used for irrigation purpose.There are about 70 shallow hand dug wells (HDW) and 30 house hold ponds (HHP) with water in the area.Out of 70 hand dug wells, 30 are lined with rock (HWR), 16 with cement (HWC) and 24 with clay (HWCL).
The house hold ponds are mostly lined using plastic sheets (Fig. 2B) (HHP) except in very few cases where cement (HHPC) is used.The hand-dug wells were constructed in 2003, while the ponds in 2004.About 10% of the wells and ponds were randomly selected for water sampling on the basis of pH, color, turbidity, taste, odor and electrical conductivity (EC) measured in the field and the one in use.At the same sites, pits were excavated in the lands irrigated by handdug wells and pond waters for depth-wise soil sampling.Samples are collected after monsoon so that water is available for irrigation particularly in the case of ponds.

Sampling
10 water samples 7 from HDW (Fig. 2A) and 3 from ponds (Fig. 3B) were collected from the pre-decided sites (Table 2).At the same sites, 30 soil samples were also collected 3 each from 10 pits excavated in the irrigated land that is irrigated by using water from the same HDW and HHP (Fig. 3C).At each site 3 samples were collected up to a depth of 100cm along profile at different intervals, 0-20cm, 20-60cm and 60-100cm.All the samples were air dried, packed and sent to laboratory for analysis.From each site, data related to the condition of the irrigable lands, vegetation cover and type were recorded in addition to assessing the accumulation of external solonchacks indicators at the surface of the soils.

Sample analysis and Data processing
Water samples were analyzed for major cations (Na + , Ca 2+ , Mg 2+ and K + ) and anions (HCO and NO - 3 using Ultraviolet spectrophotometer; and HCO 3 -and CO 3 2-using titration method.On the basis of 0.1N HCl used in the titration, alkalinity is calculated.Further, alkalinity data is used to determine the bicarbonate and carbonate ions using pH of water (Deutsch, 1997).
Where, TVS= titration volume of samples; TVB=titration volume of blank; N= Normality.
The air dried soil samples were sieved to obtain 2mm size fraction and were treated with double distilled water in the ration of 1soil: 5water ratio for measuring EC and 1soil:10 water ratio for pH using EC meter and pH meter, respectively.The aliquots were aspirated into flame photometer to analyse sodium (Na + ) and potassium (K + ) at 589 and 765.8 nm, respectively, and into AAS to analyse Mg +2 and Ca +2 at 422.7 and 288.2 nm, respectively.CO 3 2-and HCO 3 were measured by acid neutralization method using 0.1NH 2 SO 4 with phenolphthalein and methyl-orange as indicators respectively.The analysis of chloride was conducted by titration of the extracts with 0.1N AgNO 3 solution using potassium chromate.Sodium Adsorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP) were calculated as suggested by Booker Tate (1991).
AquaChem software, a fully integrated statistical package developed specifically for graphical and numerical analyses of aqueous geochemical data sets is used to process water geochemical The electrical balance (electro-neutrality) in terms of ions in water was calculated by comparing the sum of the equivalents of cations with the sum of the equivalents of the anions (Deutsch, 1997).It was done using the charge balance equations.Later these were used for all calculations involving equilibrium interactions between water and geological materials (Freeze et., 1979).The accuracy of water analysis data is estimated using balance error given by the following equation and the balance error is expressed in percentage.Water analysis normally considers a balance error acceptable when it is less than 5% (Freeze et al., 1979 andDeutsch, 1997).Apart from HWR6, all hand-dug well and pond water samples are within the acceptable level (Tables 3 and 4).

Water
Many of the water samples were not clear in appearance and odor.Four hand-dug wells were turbid and had bad odor due to re-construction and maintenance for the purpose of expansion to develop more land and in the presence of plants and other materials.Presence of algae has attributed bad taste to water in all the ponds in addition to brown color due to soil erosion.The data for both anions and cations and other parameters are provided in table 3. The electrical conductivity values for water (ECw) from HDW ranges from 570 to 1358 with a mean value of 887µS/cm and for pond from 150 to 695 with a mean value of 441µS/cm.Similarly, pH values vary from 7.7 to 7.9 for water from HDW and 6.7 to 8.3 from ponds (Table 3).
Among anions, bicarbonate dominant in the water from both hand dug-well and ponds.It, ranges from 236.13 to 669.34 with a mean of 423 mg/l in HDW and 91.77 to 194.08 (141 mg/l mean) in pond.The next dominant ion chloride ranges from 29.03 to 51.68 (41 mg/l mean) in HDW and in pond from 0.96 to 86.77 (431 mg/l mean).Similarly, NO 3 -values (as total nitrate) in HDW range from nil (in three samples) to 8.25 (2.8mg/l mean), while the same in the pond vary from 2.74 to 9.69 (6 mg/l mean).Sulphate vary from 6.2 to 53.2 (19 mg/l mean) (HDW) and nil to 10.0 (5.6 mg/l mean) (ponds).Carbonate ion concentrations on the other hand are very low and vary from 0.9 to 2.2 in HDW and nil to 2.8 mg/l in ponds (Table 3).Among cations, Ca 2+ is the most dominant cation followed by Mg 2+ , Na + and K + .As K + values being very low they are not mentioned in the table.In HDW samples Ca 2+ values vary from 80 to 136 mg/l with a mean values 104 mg/l and in pond samples ranges from 44 to 84 mg/l (mean 68 mg/l).Similarly, Na + values in water from HDW range from 4.14 to 39.1 mg/l (mean 15 mg/l) and in pond from 4.1 to 6.9 mg/l (mean 6 mg/l).Mg 2+ on the other hand varies from 14.4 to 57.6 mg/l (mean 33 mg/l) in HDW and do not shows any variation in pond where all samples shows the same value 2 mg/l (Table 2).Interestingly Ca 2+ is the dominating ion in both HDW and pond water and followed by Mg 2+ in HDW and Na + in pond water.
Further, the parameters like alkalinity and SAR were calculated using the data in table 3 and the chemical type of water was deduced using piper diagrams (Fig. 3) and given in table 4. The calculated values for Alkalinity range from 7.7 to 7.9 mg/l of CaCO 3 in HDW and from 6.7 to 8.3 mg/l of CaCO 3 in pond samples.Similarly, the SAR values for water from HDW ranges from 0.09 to 0.55 and the pond water samples ranges from 0.02 to 0.13.According to the piper diagram (Fig. 3), water samples from HDW are classified into different chemical types.They are Ca-Mg-HCO 3 , Ca-HCO 3 , and Ca-Mg-HCO 3 -Cl.Among these, the dominant one is Ca-Mg-HCO 3 type.Similarly pond water sample data suggest three different types such as Ca-HCO 3 , Ca-HCO 3 -Cl and Ca-Cl-HCO 3 but the common type being Ca-HCO 3 (Table 4).

Soil
The data generated for 24 soil samples from 8 pits are presented in table 3. Electrical  5 and 6).Among major anions, sulphate values are higher compared to others in both types of soil.It is followed by chloride and bicarbonate and carbonate.However, carbonate is almost nil in many samples.Sulphate values are lower in HDW soil (12 to 23 mg/l) compared to pond-related soil (7.2 to 52.3 mg/l) (Fig. 5) and in both the cases no clear trend is observed with depth.Chloride also shows similar trend like sulphate indicating higher values for pond-associated soil (7 to 35 mg/l) compared to HDW -associated soil (7-28 mg/l) and do not show any clear trend with depth (Fig. 5).Bicarbonate on the other hand shows downward decrease in concentration in both HDW (nil to 0.9 mg/l) and pond-related soil (nil to 36.6 mg/l).Relatively pond -related soils show higher values compared to that of HDW.Carbonate is almost nil in many samples except in one soil sample that is associated with pond in which the values range from 9 to 24 mg/l and also show lower values with depth.
In the case of major cations, calcium is the dominant ion followed by sodium, potassium and magnesium (Fig. 6).Calcium concentrations range from 2 to 29 mg/l (HDW-soil) and 7.6 to rock and clay lined are showing higher values than the water from cement lined wells.Similar trend is also observed for water from cement-lined ponds.
SAR (0.02 to 0.13) and EC (0.7 to 0.2 dS/m) values of pond water indicate that the waters needs slight to moderate degree of restriction on use.SAR values of HDW water (0.09 to 0.71) lie within 0-3 category of SAR, but based on EC, HDW water are categorized into two groups i.e. samples with > 0.7 dS/m (HWR19, HWCL17, HWR52, HWC22, and HWR53) and <0.7 dS/m (HWCL50 and HWCL6).Use of water belonging to the former group does not indicate any problem in terms of hazards of sodicity.However, slight to moderate degree of restriction is use is necessary in the case of water belonging to the later group.

Toxicity Problems
Irrigation crops dominantly grown in the area are tomatoes, hot pepper, onions, potatoes, cabbages, and maize.These crops generally do not get affected much even if there is variation in the soil and water quality parameters.However, chloride ion concentration is an important factor to be considered if treated effluent is used for irrigation.For this purpose, the concentrations should not be more than 70 to 100 mg/l, if plants are irrigated by the surface irrigation method or by a sprinkler irrigation method (Ayers and Wescot, 1976).High chloride concentration disturbs the osmotic balance between plants and soil, which affect the growth of plants because most plants are sensitive to chloride salts.The waste material being the source of chloride, it is necessary to take precautions while using waste directly or indirectly in irrigation.In the watershed, the commonly used method is surface irrigation method and in both water and soil samples chloride values are < 4 meq/l and is well within the acceptable limits for irrigation (FAO, 1985).According to FAO (1985) chloride concentrations in surface irrigation should be below 4 meq/l and 3 meq/l for sprinkler irrigations.Thus there is no restriction in the use of water and soil in the area in terms of chloride.

Soil Sodicity
The SAR values for hand-dug wells, ponds and soil range from 0.19 to 1.27 while the exchangeable sodium percentage (ESP) results are below 1% (Tables 5 & 6).Soils with SAR values greater than 13 are usually considered sodic.According to USDA soil classification, soils showing electrical conductivity values <2000 µS/cm (at 25 o C), ESP <15%, SAR <13 and study area is dominated by Adigrat sandstone lithology.It forms part of the geological succession of northern Ethiopia and belong to Mesozoic age.Stratigraphically, the area is composed of basement Precambrian low grade metamorphic rocks and overlain by the younger Paleozoic Enticho sandstone and Adaga Arbi glacials units; Mesozoic Adigrat sandstone, Antalo limestone, Agula shale, Amba Aradom sandstone and alluvial deposits.Adigrat sandstone occupies the highest levels in the topography and is overlie the Paleozoic sedimentary rocks.The outcrops are mostly found in the south and southeastern parts of the watershed.There is a prominent escarpment in the area which consists of metamorphic rocks in the middle and colluvial deposits near the base.1.4.Soil Based on traditional classification, the dominant soil types of the study area are categorized as Hutsa (sandy), Baekel (loam) and Walka (clay), respectively.Major soils of the irrigated areas of the watershed are Vertisols, Cambisols, Leptosols and Alluvial soils.The dominant plant species in the watershed are bushes and shrubs, Acacia saligna, Dodonea visscosa, Eucalyptus species and trees like remnants of Olea europeana.Regenerated species are available in the plantation and area enclosures.Eucalyptus species, Acacia saligna and Dodonea in the plantation sites are part of biological soil and water conservation.These were introduced during the initial stages of the forest development program.
for pH and electrical conductivity (EC) in the geochemical laboratory, Department of Earth Science, Mekelle University.Major cations were determined using both Flame Photometer and Atomic Absorption Spectrophotometer (AAS); Cl -, SO 4 2- diagram is prepared to represent and compare water quality data in the area.The soil samples on the other hand were treated with simple statistics such as mean, correlation etc.

Figure 3 .
Figure 3. Piper diagram for both hand dug well (A) and pond (B) water sample data.
conductivity values at 25°C for soil samples from HDW irrigated land varied from 17 and 1445 µS/cm and the same in the soil from pond water irrigated land ranges from 51 to 1734 µS/cm.EC values for soils from hand dug-well water irrigated land in general are lower compared to soil from pond water irrigated land.In the case of pH, soil related to HDW is slightly acidic (6.5 to 8.0) (Fig4A & B) compared to pond related soil which is slightly basic (7.5 to 8.2) (Fig 4C & D).Interestingly, both EC and pH values decrease downwards with depth (Tables Note: (a= from 0-20 cm depth, b = 20-60 cm, c = 60-100 cm; HHP= Pond plastic lined; HHPC = Pond cement lined).

Figure 4 .
Figure 4. Depth-wise variation of EC, pH in soil related to HDW and Ponds.

Figure 5 .
Figure 5. Depth-wise variation of sulphate and chloride in soil related to HDW and ponds.

Figure 6 .
Figure 6.Depth-wise cations variation in soil related to HDW and ponds.

Table 3 .
Hand dug well and pond water sample data, Gergera Watershed.

Table 4 .
Calculated values for alkalinity, SAR and chemical type of water for both HDW and pond samples, Gergera Watershed.

Table 5 .
Soil data from hand dug wells, Gergera Watershed.