Towards a Better Design of Micro-Catchment Water Harvesting Schemes. - As pressure on land is increased more marginal areas being used for agriculture. -One technique to help alleviate water shortages and to gain increased yields is water harvesting, which is the collection of runoff for productive use -Instead of runoff causing erosion and being lost to rivers and oceans it is harvested and utilized being both a method of soil and water conservation By Dave Best

Direct rainwater harvesting from micro-catchment schemes (within field catchment systems) Overland flow harvested from a short catchment. 1-30 metres. Concentration of surface runoff from a catchment into lower lying cropped area. Ratio of catchment to cultivated area- from 1:1 to 20:1. Offers a passive, low cost means of increasing the amount of water available to crops *read from here. In this review I shall be concentrating on direct rainwater harvesting systems from micro-catchment schemes, which are within-field catchment systems. -main characteristics of micro-catchment schemes include that overland flow is harvested from a short catchment length usually between 1 and 30 meters. This system generally involves controlled concentration of surface runoff from a catchment into a smaller lower lying cropped area. The accumulated runoff is stored in the soil profile, normally with no provision for overflow. The ratio of catchment to cultivated area may range from 1:1 to 20:1 *see slide for last

Advantages over alternative irrigation schemes. Systems relatively easy to maintain and operate. Simple and easy to construct Once constructed little maintenance is required. Runoff water low salt content advantages associated with micro-catchments over alternative irrigation schemes. The systems are relatively easy to maintain and operate and are also relatively safe from failure. They are simple and easy to construct using local materials and manpower, and once constructed little maintenance is required. The runoff water has a low salt content, as it does not have to be transported, or pumped which reduces soil salinity.

Examples of micro-catchment water harvesting systems *slide intro 1) often used for trees or grass. Forms a closed grid of diamond shapes or open-ended V shapes bordered by small earth ridges, with infiltration pits. Suitable for uneven terrain. 2) These can also be used for trees. Formed with earth bunds along contours which are spaced at 5-10 metres apart. Often used for planting on a large scale, however not suitable for uneven terrain. 1) Negarim micro-catchments 2) Contour bunds

3) Semi circular bunds 4). Contour ridges 3)Can be used for rangeland, fodder and sometimes trees. Formed by semi-circular earth bunds which are lined along a contour in a staggered formation. Difficult to mechanize. 4) often used for crop production. Small earth ridges on a contour with furrow upslope and cross ties. The ridges are generally 1.5-5m apart, with the area between the ridges forming the uncultivated catchment. 4). Contour ridges

Design Considerations To improve catchment efficiency runoff can be increased, and infiltration and interception reduced. Balance between the efficiency of a catchment in producing runoff and its susceptibility to erosion Increasing runoff by clearing slopes of vegetation and compaction of surfaces may lead to greater erosion with longer slope lengths and steeper gradients. *read this first An important factor in the design of micro-catchments is that they are only effective when rainfall is sufficiently intense to create surface flow.

Determination of Catchment yield. Rainfall Characteristics. average annual rainfall, peak rainfall intensity, and minimum expected annual precipitation. Physiographic factors. Infiltration Capacity. *read these 1-The rainfall pattern is important as it is affects runoff in the micro-catchment, with a larger proportion of the high intensity rainfalls going towards surface runoff 2-Physiographic factors must be investigated –these include runoff producing potential, soil surface condition (vegetation, cover, stones, crust), water retaining capacity of the soil in the root zone profile and the gradient and evenness of the slope. all of which contributing to the runoff threshold coefficient 3- Infiltration Capacity. Which ca be affected by soil moisture content, macropores in the soil (from worms, decaying organic matter etc), and soil compaction

Catchment : Cultivated area Rainfall Runoff Crop water requirements. *just read this In the design of a micro-catchment system it is important to determine the required ratio between catchment and cultivated area, however this is only possible in cases where rainfall, runoff and crop water requirements are known. For each species of crop there will be an optimum size for the micro-catchment in any particular area, and for this the water demands of a plant must also be assessed

Catchment : Cultivated area -calculations……. Water Harvested = Catchment area x Design rainfall x Runoff coefficient x Efficiency factor. *read this at end As not all runoff can be efficiently utilized (because of deep percolation losses, etc.) it must be additionally multiplied with an efficiency factor

From this we can produce the equation, Crop water requirement x Design rainfall = Catchment Area Design rainfall x Runoff coefficient x Efficiency factor Cultivated Area

Example. Crop Water Requirement = 525 mm Design Rainfall = 310 mm Runoff Coefficient == 0.5 Efficiency Factor == 0.75 C = 525 – 310 = 1.85 Ca 310 x 0.75 x 0.5 Here is an example of a calculation of the Catchment area : Cultivated area ratio for a crop of 110 day sorghum grown in a semi arid area using a micro-catchment. i.e. The catchment area must be roughly 1.85 times the size of the cultivated area.

Surface treatments Mechanical treatments (smoothing and compacting). Colloidal dispersion methods (slaking, sodium). Hydrophobic applications (water repellents). Surface binding materials (cementing and sealing) Surface covering (asphalt, rubber and plastic) *Read from here. Many water harvesting catchment surface treatments for increasing runoff have been tested throughout the arid and semi arid regions of the world. *now slide Among these treatments, probably the cheapest involve clearing, smoothing and compaction making them economical for crop production. Chemical treatments, however have a shorter effective life of 2 to 5 years, although at the same time urface coverings with impermeable materials like plastic, and other thin sheetings are susceptible to damage by the elements

Experiment testing different techniques for catchment lining on a plantation of Jujube (Zizyphus mauritaina) Shallow conical micro-catchments of 1.0m radius were constructed around the plant *read title then here Jujube is an important fruit crop of the Indian arid zone generally grown under rain fed conditions Catchment surfaces were lined with different materials including polythene bags, newspaper, stone and marble pieces. The soil moisture status and the saplings establishment success was monitored.

The soil moisture depletion pattern in various treatments in a 90cm profile The Graph shows effect of mulching with the variation of soil moisture in 90cm profile in various treatments after this time, showing that stone and marble treatments maintained considerably higher moisture even after 60 days of dry period. Polythene and paper treatments also maintained higher moisture contents as compared to control. Also resulted in inc growth, leaf area, no of branches and mortality was reduced.

Jujube *read here first -The Demonstrated the micro-catchments ability to help in establishment, and also improves growth. -The lining of the catchment with mulch materials proved beneficial in both the quantity and conservation of harvested water. Mulches can increase infiltration rate by intercepting and absorbing raindrop impact, and impeding lateral flow of excess surface water. Materials like stone, marble, and wood chips which are generally cheap and readily available in arid areas, are durable and unlike other chemical treatments do not degrade the soil.

Using flow models. i.e HILLFLOW *from here To take things further we can quantitatively assess hill slope hydrological processes, a number of modeling systems have been developed. The modeling system accounts for the hydrological processes of interception, evapotranspiration, infiltration, soil-moisture movement surface runoff, subsurface storm flow and stream-flow discharge, with relevant interactions also included. An example of one of these models is HILLFLOW. The kinematic flow models which I wont get into now can be used with regard to overland flow together with a physically based representation of the infiltration process from which are derived general relationships that govern the occurrence of flows.