This document discusses water harvesting techniques for crop and pasture production in arid and semi-arid lands. It defines water harvesting as collecting runoff for productive purposes. It then classifies and describes various water harvesting techniques including micro-catchment systems, external catchment systems, and floodwater farming. Design criteria such as calculating crop water requirements and factors that influence requirements are also covered.

1. WATER HARVESTING & MANAGEMENT (WHM) FOR CROP & PASTURE PRODUCTION FFA Regional Training March 22 – 24, 2011 Mombasa, Kenya. Presented By: Kimeu P. M & Mutiso J.W.

4. Classification of Water Harvesting Techniques Water Harvesting Rainwater Harvesting (Local Source) Floodwater Harvesting (Channel Flow) Rooftop Harvesting (collection from rooftops) Runoff Harvesting (overland/rill flow) Deep Ponding (storage) Water Supply Deep Ponding (storage) Water Supply Soil Storage Plant Production Runoff Farming** Micro-Catchment Systems (Short slope catchment techniques) External Catchment Systems (Long slope catchment techniques Deep Ponding (storage) Water Supply Soil Storage Plant Production Floodwater farming = Water spreading 1. 2. 3. Sub Divisions Main plant production categories Productive use* Storage Category of WH system by source NB: * Water supply systems (i.e. ponded water) used for a variety of purposes, mainly domestic and stock water but also some supplementary irrigation ** ‘Farming’ in ‘’Runoff farming’’ broadly used to include trees, agro-forestry, rangeland rehabilitation, crops etc.

9. Negarims Examples of main WH systems Contour Bunds Semi-Circular Bunds Contour Ridges

10. Trapezoidal Bund Contour stone bunds Permeable rock dams Water spreading bunds

13. DESIGN CRITERIA

17. Crop Factors (Kc) CROP Average Kc per growing season Cotton 0.82 Maize 0.82 Millet 0.79 Sorghum 0.78 Grain/small 0.78 Legumes 0.79 Ground nuts 0.79

22. Because the objective of rangeland & fodder grown in ASAL under WH systems is to improve performance, within economic constraints, and to ensure the survival of the plants from season to season, rather than fully satisfying water requirements!

26. 7. Return period T (in yrs) can be easily derived once exceedance probability P (%) is known from the equation: T= 100 (years) P E.g. T 67% = 100/67 = 1.5 (years)

29. Design Model for Catchment : Cultivated Area Ratio C:CA calculation for crop production systems Rule: WATER HARVESTED = EXTRA WATER REQUIRED Interpolating the above we obtain CWR – Design Rainfall________________ = C Design Rainfall X Runoff Coeff X Eff Factor CA N.B Runoff coeff is proportion of rainfall which flows along ground as surface runoff (ranges between 0.1 and 0.5) Efficiency factor takes to account the inefficiency of uneven distribution of water within field as well as evaporation losses and deep percolation (ranges between 0.5 and 0.75)

32. Example: Microcatchment system (Negarim microcatchment) for trees MC = RA x WR - DR DR – K - EFF where: MC = total size of microcatchment (m 2 ) RA = area exploited by root system (m 2 ) WR = water requirement (annual) (mm) DR = design rainfall (annual) (mm) K = runoff coefficient (annual) EFF = efficiency factor As a rule of thumb, it can be assumed that the area to be exploited by the root system is equal to the area of the canopy of the tree.

33. Example: Semi-arid area, fruit tree grown in Negarim microcatchment Annual water requirement (WR) = 1000 mm Annual design rainfall (DR) = 350 mm Canopy of mature tree (RA) = 10 m 2 Runoff coefficient (K) = 0.5 Efficiency factor (EFF) = 0.5 Total size MC = 10 x {(1000-350)/(350 x 0.5 x 0.5)} = 84m 2 As a rule of thumb, for multipurpose trees in ASAL, the size of the microcatchment per tree (C and CA together) should range between 10 and 100 m 2 , depending on the aridity of the area and the species grown. Flexibility can be introduced by planting more than one tree seedling within the system and removing surplus seedlings at a later stage if necessary.

37. Negarims under construction in different parts of Kenya Taita Taveta district Taita Taveta district Taita Taveta district Turkana district

40. V-Shaped micro-catchments Sometimes, open-faced V shaped MC may be constructed to allow surplus water to overflow

44. Zai pits constructed in Kilifi district

48. Contour bunds shortly after rains in Garissa district

49. Illustration of contour bunds

51. Contour ridges field layout

52. Contour ridges in Marigat, Baringo

54. Construction: Stake out using survey equipment the tip of control contour and using string stake the continuous contour

58. Source; FAO Water Harvesting Manual 2

59. Layout of trapezoidal bunds and a completed structure in Makueni district

60. Trapezoidal bunds Field arrangement of a large catchment area

61. Recommended dimensions of one TB unit %slope Length of base bund (m) Length of wing wall (m) Distance between tips (m) Earth work per bund Cultivated area per bund (sqm) 0.5% 40 114 200 355 9600 1.0% 40 57 120 220 3200 1.5% 40 38 94 175 1800

62. Consequences of poor compaction of TBs

64. Intake from stream Field layout of water spreading bunds

67. Permeable rock check dams for gully control and catchment conservation in a water project site, Makueni district

74. General, slope-dependent dimensions of Fanya Juu terraces: Slope, % VI, m HI, m Width, m Depth, m Channel area, m 5 1.00 20 0.50 0.50 0.25 10 1.35 14 0.50 0.55 0.28 15 1.73 12 0.60 0.55 0.33 20 1.80 9 0.60 0.60 0.36

76. Measuring slopes and marking contours using line level NOTE: The gully in the left…. What do we do when the fanya juu has to cross such an area?

77. DEMONSTRATIONS: Field assembly of an A – Frame and field calibration as well as usage of the A – Frame to mark contours A – Frames very precise but not recommended for layout of terraces in large areas….. WHY??? Can A- Frame be used to measure/determine slopes??

78. What are these people doing?

79. Discuss the above photos

82. Which soil & water conservation structure is this??