Measuring and monitoring soil erosion for soil conservation and soil protection in Israel Yoram Benyamini Ministry of Agriculture (SERS), Israel / University of Amsterdam, Netherlands
[email protected] Abstract Erosion measurements are usually hard to carry out and are expensive in resources. Measurements of runoff are much easier, less money demands. Authorities in Israel are more aware of the shortage of water than of the problem of erosion, so they are willing to invest in developing water resources much more than in erosion measurements. The climate conditions in Israel are characterized by a rainy season that starts often in October and ends in April, often with very high intense rains after dry periods. Such rains that fall on bare soils are the recipes for high erosion rates. Difficulties rise related to the erosion process evaluation as we suffer from lack of available data. Looking at the erosion process that is mainly the result of raindrop impact and the water flow, allows us to concentrate more on water measurements than on erosion measurements. So, we are trying to use the widespread runoff and rainfall measurements to decide the appropriate and recommend soil conservation means to apply. The selection of soil conservation methods and means is based on the rainfall – runoff – erosion relationship in each location, taking into consideration soil type, topography and the crops. Methods and means that reduce runoff, increase water infiltration into the soil and collect overland flow water by reservoirs, are those that are minimizing and eliminating soil erosion. The separation of the lands into three different areas: agriculture, non-agriculture and near urban areas helps us to select the suitable soil conservation method and mean. The conservation means are divided into categories such as: agrotechniques, crops, engineering and chemical. One or a combination of a conservation method and mean can be adjusted to the needs. The influence of the conservation methods and means on the overland flow together with the available knowledge of these influences on the erosion, allows us to choose the best combination method to reduce and eliminate erosion. The long term experience and the knowledge of the rainfall pattern together with the physical processes of soil crust formation brought us to the techniques that we apply in the fields. Examples of conservation mean are given in the paper: 1) In non-agricultural lands, the reduction of runoff and erosion by afforestation method in wild hills at the south part of Israel; 2) In agricultural lands, adjusting minimum tillage methods in field crops. The erosion measurements are not ignored, they are carried out mainly in research projects, but the water flow measurements are carried out widely by governmental agencies as well as by other institutes. So, the decision in selecting soil conservation methods and means is mainly based on the water flow measurements and rainfall analysis. 147
1. Forward Erosion measurements are usually hard to carry out and are expensive in resources. Measurements of runoff are much easier, with less money demands. Authorities in Israel are more aware of the shortage of water than of the problem of erosion, so they are willing to invest in developing water resources much more than in erosion measurements. The purpose of this paper is to show that we can use runoff data and rainfall analysis for choosing soil conservation means to reduce and eliminate erosion. It is based on the work brought by Agassi et al. (1996), Agassi et al. in Issar and Resnik (1996). 2. Climate Israel is located on the east side of the Mediterranean Sea. This area suffers from extreme variations in annual rainfall amount, in storms distribution within each year and in the rainfall amount that changes from the north to the south and from the west to the east. The rainy season spreads from October to April, and most of the rains fall between November and March. The average annual rainfall varies from 50 mm in the south to 1200 mm in the north, and from 500 mm in the mid west to 30 mm in the mid east. The change in the annual rains is spread over 400 km south to north and 50 km west to east. This climate brought problems of water shortage that is the limited factor for growing crops. The rains are characterized by high intense storms which come often after dry periods mainly in the autumn after a long dry summer, or after dry periods among rainstorm events. 3. Soils Most of the Israeli soils are vulnerable to erosion and to loose their fertility. The Loess (Calcic Haploxeralf), the Hamra (Typic Rodoxeralf), the Rendzina and the mountain soil types are continuously affected by erosion processes. High intense rainstorms that fall on dry soils produce runoff and erosion. Usually, the soils are bare after the long drying period of the spring and the summer. The topography varies from moderate slopes in the plains to the steep slopes in the mountains. For easier evaluation in choosing soil conservation means we divided the lands into 3 categories: Agriculture lands, Non-agriculture lands and lands near the urban areas. The agricultural lands are the most susceptible to erosion. The nonagricultural lands have usually natural protection, but as a result of over-grazing, and other human activities, the land became under risk of erosion. The land near the urban areas is on the focus of cities developments (real estate) potentials and treated accordingly – left uncultivated without conservation means. 4. Erosion As a result of the climate, soil and human activities, there are problems of erosion. Erosion occurs at the locations where the soil is taken from, at the location where the soil mass is transferred through and in the deposit locations where the sediments are settled down. Where ever we go we see the signs of erosion. During and after rainstorms we see the gullies and the ditches in the upper and along the slopes, and 148
in downslope areas covered with deposited soil. In order to repair and eliminate these destructions we need to evaluate the intensity of the erosion process and the reasons that cause it. 5. Erosion measurement General In a matter of fact, the measurements of erosion in Israel are not in high priority. It is carried out only in few research projects and not as a routine work around the country. In general, erosion is measured in the field mainly at a research scale. In Israel, the activities of measuring erosion are made now mainly at universities: Laronne L., Inbar M., Lekach J., Lavee H, and by few others. The trend to use models in erosion evaluation is attractive. As a result, potential soil erosion rates are typically modeled from empirical and functional relationships driven by factors related to soil properties, climate, and landscape position. These models do not yet represent the actual soil erosion losses and only provide estimations of potential erosion under specified climatic conditions. Without repeated field-based measurements, there is currently no way to determine accurate erosion losses. The causes of erosion Soil erosion by water occurs when bare-sloped soil surface is exposed to rainfall, and the rainfall intensity exceeds the rate of soil intake, or infiltration rate, leading to soil-surface runoff. Soil erosion can occur in two stages: 1) detachment of soil particles by raindrop impact, splash or flowing water; and 2) transport of the detached particles by splash or by the flowing water. Therefore, soil erosion is a physical process requiring energy, and its control requires certain measures to dissipate this energy. Many researchers found a relation between erosion and runoff. Among them, Pruski and Nearing (2002) showed good relationships between runoff and erosion using different models. Keppeler et al. (2003) showed the connection between the runoff and sediment load in the river during forest management and changing the forest cover in time. 6. Water measurements The measurements of rain and water flow are widespread in Israel. The water resources in Israel are very limited. So, much of the investments made during years were in measuring rainfall and over-land water flow as well as for developing methods to increase the water availability. A number of governmental and nongovernmental institutes are taking part in the above measurements. Rainfall measurements techniques use all the available types of devices, from a simple cumulative rain gauge to the sophisticated Radar measurements. Measuring water runoff also use different types of devices and ways of after-rain events flows evaluation. The two data types, rain and water flow are analyzed and formulated together to enable us to evaluate their influence on the erosion.
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7. Alternatives to Erosion data Practices and methods that control and minimize the energy of the water, reduce and even eliminate the development of erosion. The measurements of water flow as the result of different conservation means and method that were applied in the field lead us to the possibility to choose the best conservation treatment that controls the flow. The water flow control resulted in diminishing the erosion process. Erosion measurements in Israel are mainly of transported sediments that are made in few locations, mainly under watershed projects (Laronne, 2003). The large amount of water flow and rainfall measurements, which are still going on, led us to the possible good water flow prediction. These measurements and predictions with the knowledge of the soil erodibility can replace erosion measurements for soil conservation means decisions. 8. Water and erosion control 8.1 Yatir case study – non-agricultural land The objectives of this project were to reduce the erosion, to control the flood and to cover the hills by green vegetation. The watershed, which is located on the foothills of Hebron Mountains, was chosen to be under an afforestation method to fulfill these objectives. The area is hilly under annual rainfall of 250 – 350 mm, the slopes have shallow calcareous soils, covered with sporadic shrubs. The flow of water after rainfall events concentrates along the main waterway and was found to have a dark color indicating that it contains sediments. The average annual rainfall is not sufficient to supply enough water to trees. So, the main ideas were: 1. Blocking the flow on the slope by parallel ridges. 2. Plant trees behind the ridges. 3. Leaving bare spaces between the ridges. The rain will produce crusts on the bare soil surface, and runoff water will flow and accumulate behind the ridges around the trees. 4. Calculating the space between the ridges depends on the soil slope and on the expected rainfall. 5. Space of 10 m between the ridges will supply about 1000 mm for each tree. (4m x 10m =40 m2, 250 mm rain – 60% evaporation = 100 mm x 40 m2 = 4 m3 for each tree). The results will be: 1. Hill slopes with forest cover, capable of surviving in these conditions. 2. Minimum water flow will leave the area. 3. The water will transport small amount of erosion. In the first few years, sediments will be moving by the water but will accumulate behind the ridges. Later, shrubs will start to cover the bare soil between the ridges and the runoff supply will be decreased. The next step will introduce sheep and goats under control grazing to reduce the vegetation cover and to return the runoff capability. 150
This project is accompanied by measurements of runoff and sediments, but no measurements of erosion were carried out ahead of the project. Nine pictures show the activities of the project:
1. Runoff with sediments
2. Measuring device
4. Earthwork and the type of the natural cover
6. One year after the work and after rain event
8. Aerial photograph of part of the watershed.
3. Earthwork and signs of erosion
5. At the end of the work
7. Ortho-photo of the watershed
9. Conditions after 6 year
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Results For details see Safriel et al. (2003). Figure 1 and figure 2 show the correlation between the erosion – sedimentation and the runoff - water flow.
Fig. 1 Discharge hydrograph and sediment concentration from the lower newly afforested watershed on 20-21 December 2002. (red line shows time of sampling)
Fig 2. Discharge hydrograph and sediment concentration from the upper unafforested watershed on 20-21 December 2002. (The red line shows the time of sampling). Also, Figs 1 and 2 show the influence of afforestation on the runoff and the erosion.
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Fig 3. Unit discharge hydrographs (discharge per unit of drainage area) from the upper nonafforested and from the lower, newly afforested watersheds on 20-21 December 2002.
The results also show the influence of the forest on the runoff and the erosion and show that the soil treatments – the conservation means and the forest cover reduce the runoff and the erosion as well. This knowledge allows us to design different types of conservation means. The later measurements will let us the possibility to assure this design. 8.2 Dalia No-Till project – agricultural land This project is carried out on agricultural fields for field crops in a hilly area in the center of Israel at Ephraim Mountains. The soil was cultivated by machinery for about 40 years. The custom tillage was plowing with discus, smoothing by frame, seeding and compacting. Within the years the farmers realized that their soil is disappearing. It came to the situation that the thickness of the soil in the upper slope was about 5 cm deep. The depth of the soil was changed from about 50 cm deep to about 5 cm deep. After each storm the farmers could detect many gullies along the fields. Lately, at the end of the crop season, the farmers even lift back soil from the bottom of the slope and spread it out on the surface. A high amount of erosion is moving down the hills. Even without any measurements one can realize the big problem. Measurements of water flow in the main waterways (Dalia creek) in the area showed high water discharge with fast response to the rainfall events. The solution for this problem, without abandoning the land was to introduce the No-till technique (Lal, R. 1984a). This tillage method is described as one that keeps the residue and the stubble mulch after harvesting and seeding the new crop 153
into the left mulch from the previous season. The seeder is quit a heavy tool that needs to cut down the straw and insert the new seeds into the ground. The appropriate machinery was chosen according to the experience in other locations with similar conditions - stony and shallow surface. Follow up after introducing this tillage method, mainly qualitative work, photography, calculations and yield measurements were made. The results show that already 3 years after starting the No-till cultivation method, the soil was enriched with organic material. Walking on the surface gave soft feeling. No gullies were developed and no erosion could be distinguished at the bottom of the slope. The results show the elimination of most of the erosion process and may even improve the water balance in the soil. Some photographs concerning mentioned tillage method are shown below. Photo 3 shows the problem of this tillage method; as a result of much organic residue the seed are sometimes uncovered.
Photo 2. No-till drill.
Photo 1. Custom tillage with erosion signs Photo 3. Soil surface after sowing.
Conclusions 1. It is possible today, according to the wide knowledge in soil erosion process, water flow data and rainfall data to apply conservation means with minimum erosion measurements. 2. Dividing the lands into 3 categories enables us to adjust easier suitable methods of soil conservation means and method. 3. Measuring erosion is difficult and expensive so we can reduce very much the investment of measuring erosion and use water flow measurements instead.
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References Brazier R.E., A.J. Parsons, J. Wainwright and D.M. Powell. 2003. Observed controls of runoff and sediment yield in semi-arid environments from the hillslope to the catchment scale, Geophysical Research Abstracts, Vol. 5, 04179, 2003 European Geophysical Society. Fergus, and D.E. Walling, eds., Erosion and Sediment Transport Measurement: Technological and Methodological Advances. International Association Hydrological Sciences Redbook Series. Oslo. In press.Lal, R. 1984a. Mechanized tillage systems effects on soil erosion from an Alfisol in watersheds cropped to maize. Soil and Tillage Research 4:349-360. Laronne, J.B., Y. Alexandrov, N. Bergman, H. Cohen, C. Garcia, H. Habersack, M.P. Powell, and I. Reid. 2003. The continuous monitoring of bedload flux in various fluvial environments. In J. Bogen, T. Keppeler E., Lewis J and Lisle T. 2003. Effect of Forest Management on Stream flow, Sediment Yield, and Erosion, Caspar Creek Experimental Watersheds. First Interagency Conference on Research in the Watersheds. Benson, AZ, 27-30 October 2003. Pruski, F.F. and Nearing M.A. 2002. Runoff and soil loss responses to changes inn precipitation: a computer simulation study. J. Soil and Water Conservation. 57(1):7 – 16. Safriel U. N., Moshe I., Berliner P., Novoplansky A., Getker M. and Arbe S. 2003. Monitoring and Evaluating Afforestation of a Semi-Arid Watershed in Yatir Region, Israel. Annual Scientific Report Middle East Watershed Monitoring and Evaluation Project.
Conservation implications of climate change: soil erosion and runoff from cropland. A report from the Soil and Water Conservation Society
Soil and Water Conservation Society 2003. Agreement No. 68-3A75-2-98.
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