WATER HARVESTING Rabi H. MohtarPurdue UniversityWest Lafayette, Indiana, USA
Dryland Agriculture of South Tunisia • Southeastern Tunisia receive mean annual rainfall less than 200 mm. Rainfed farming is supported mainly by the water harvesting techniques. • If: rain < 250 mm irrigation is necessary for crop production • Problem is characterizing the existing WHS and assess the impact of these systems on the rural communities
Water Harvesting in the Dry Upland Areas of Lebanon • Rainfall less than 200 mm per year • Poor farming communities • Alternate crops such as cherries is being grown as an economical crop but it requires additional water • Problem is reservoir siting to provide supplemental irrigation
Water Harvesting Classification • Water harvesting techniques are classified into: 1) runoff water harvesting that makes use of runoff as it is collected, eliminating the storage requirement, like "jessour"; 2) floodwater harvesting and spreading or spate irrigation with diversion dikes called "mgoud"; and 3) runoff water collection and storage in reservoirs of variable capacities for drinking, animal watering and irrigation purposes.
Jessours in the Matmata moutains. A hydraulic unit made of three components: the catchment area, the terrace and the dyke. The jessours are used to bank the runoff from the mountain chain.
Tabias newly installed in the piemonts areas. The dyke (tabia, sed, katra) is a barrier destined to block the sediments and runoff waters. Its body is made of earth equipped with a central and/or lateral spillway and one or two abutments (ktef), assuring the evacuation of the excess water. It has a trapezoidal shape (length: 15-50 m, width: 1-4 m, height: 2-5 m). The tabias are now widely used in the piedmont areas where the fruit trees (mainly olive and almond) groves are gaining larges areas at the expense of grazing lands.
Recharge well in combination with a small gabion check dam. small check dams or spreading structures for diverting runoff waters. In some cases, recharge wells are used also in combination with gabion units for aquifer recharge. Recharge well
Terraces for afforestation in Matmata. constructed on steep slopes. They are formed of small retaining walls made of rocks to slow down the flow of water and control erosion.
Feskia or Cisterns used for drinking and animal watering. a cistern is a hole dug in the ground with a gypsum or concrete coating to avoid vertical and lateral infiltration
Gabion structures We distinguish two groups: • Gabion with recharge well • Gabion without recharge well
Gabion with recharge well Based on the functioning of these structures, it is suggested to adapt the reservoir system, which exists already in the model. However, the parameters have to be modified: • RES_K (Hydraulic Conductivity of the reservoir bottom): Since the reservoir is meant for long term storage of water, RES_K is very low (varies between 0 mm/hr (Min) et 1 mm/hr (Max) ). However, our structures have an opposite role, to enhance the infiltration of water into the soil and the aquifers. So, the permeability has to be largely higher than 1 mm/hr.
Changing the lower limits • Monthly outflow data (lachures, pompage…): The data is presented in the form of a table of two rows and 12 columns (Min and Max daily outflow data for the month)…. For the gabion, it is suggested to establish a direct relationships with the rainfall Resv = f(pluie). That is when there is sufficient rainfall the reservoir will be full and the water should be infiltrated and/or evaporated within few days (1 to 4 days).
The number of gabion units to be inserted The Reservoir system limits the number to one unit by sub basin. However and for our case, these units are normally installed along the river beds with variable spacing (minimum 200 m). =Multiplying the number of units • Structures dimension The dimensions are variables Use average values.
1 4 Oued 1 2 Rainfall 3 Flow direction 4 • Functioning mode The units are installed in a gradoni form along the river (wadi). The outflow passes to the unit downstream …. Depending on the rainfall shower (intensity, volume, spatial coverage), one to many units will be considered.
Gabion without recharge well • The same remarks are to be taken into consideration with only one parameter: The hydraulic conductivity is much lower. For example, the average hydraulic conductivity at oued Hallouf gabion unit is 180 mm/hr.
Digue en terre Jesr déversoirs (menfess) Jessours • These structures are encountered in the mountains. They could be inserted as points (X,Y,"Z") (table dbf) The average infiltration rate of the jesser of Amrich is 120 mm/hr).
Tabias The tabias are techniques encountered in piedmont and flat areas. • They are supplied in two ways: 1. Direct interception of runoff 2. Flood spreading
Direction d'écoulement Canal d'évacuation au niveau du sol Bassin de retenu Banquette latérale Direction d'écoulement Banquette principle Evacuation des trop-pleins par seuil en pierres Direction d'écoulement Tabias: Direct interception The system is supplied directly from the adjacent impluvium.
Légende: 1 Oued hallouf 2 affluent de l'oued hallouf 3 canal central des oueds 4 banquette de diversion 5 sens de l'écoulement dévié 6 tabias plantées d'oliviers 7 canaux d'arrivée d'eau (mgoud) 8 déversoirs principaux 9 déversoirs secondaires 2 1 3 4 7 5 9 88 7 6 5 9 88 6 7 9 88 Tabias: Flood spreading The system is located next to a wadi and the flood is directed via a diversion dike (mgoud). Note: When the diversion dike is made of earth it acts as fusible; it breaks down when the flood is violent to prevent damages on the cultivated lands. Tabias on Oued Hallouf
GIS Database Satellite Imagery Topography Soil Permeability Precipitation Land Cover DEM Subwatersheds / Streamnetwork AHP RSI / Potential Site Ranking WMS Potential Runoff Fig. 1: General flow chart of the developed methodology.
Level 1 Goal Level 2 Major Decision Criteria Level 3 Sub-Criteria Level 4 Attribute Classes high medium low high medium low Potential Surface Runoff Potential Storage Topographic Characteristics RSI favorable Soil Characteristics unfavorable high Land Cover medium low Fig. 2: Decision Hierarchy Structure.
Sub-scenario I-A - Potential Surf. Runoff; high RW - Topography ; low RW Scenario I - Potential Storage; high RW - Land cover; low RW Sub-scenario I-B - Topography ; high RW - Potential Surf. Runoff; low RW SCENARIOS Sub-scenario II-A - Potential Surf. Runoff; high RW - Topography ; low RW Scenario II - Land Cover; high RW - Potential Storage; low RW Sub-scenario II-B - Topography ; high RW - Potential Surf. Runoff; low RW Fig. 6: Main scenarios and sub-scenarios of the study
Required ArcView themes • Elevation map Grid (DEM) • DEM Mask (Watershed of oum Zessar) • Land use map Grid • Soil map Grid • Weather (temperature, precipitation, Solar Radiation, Wind Speed, and Relative Humidity) Weather Generator if not available