Planning and management of irrigation water

1. Planning and management of irrigation water Pieter van Heerden With material from: Prof Sue Walker, Dr Dirk Raes, Mr Felix Reinders

2. Introduction

4. Background to irrigation water planning Need to know about: • Crops and their characteristics • Soils and their characteristics • Climate / weather of the area • Engineering considerations • Economic considerations

5. Crops Growing characteristics • Suitability for climate • Suitability for soil • Planting date • Growing period • Height • Density / foliage cover • Rooting characteristics • Soil water extraction • Adaptation to water stress situation

9. Soils • Texture • Structure • Depth • Chemistry • Infiltration rate

16. Climate / weather • Average, maximum and minimum temperatures • Humidity • Frost • Wind • Sunshine / radiation • Rain, season • Hail, storms

20. Engineering • Suitable and affordable systems • Suitable for crop • Suitable for soil • Losses between pump and rooting area

22. Economic considerations Markets Funding Profit Availability of input material

24. Irrigation scheduling

25. Irrigation scheduling • Getting the right amount of water to a crop at the right time This means that you should: • Know your crop and its characteristics • Know your weather and its influence crop water requirement • Know your soils, their water holding capacities and their limitations and or problems • Know your irrigation systems, their applications and limitations • Estimate expected irrigation requirements within the scope of source, system, crop, soil management limitations • Do real-time irrigation management and adapt your plan if necessary

26. Soil, Crop, Atmosphere Continuum

27. Simplified soil-crop-atmosphere continuum Water moves from soil to crop roots: low water content and high salinity changes osmotic potential: less water for crop. Water moves through roots to stem: movement restricted: high atmospheric demand cause temporary wilting. Water moves through stem to leaves: as above. Water vapour from leaves during transpiration; speed depends on: Weather situation Soil water content Crop characteristics

28. Determination of crop water use • Direct measurement • Not easy or cheap • Used mainly for management and less so for planning • Relative to a known quantity • A-pan and other evaporation pan approaches • Calculation approach • Penman-Monteith and others

29. Penman-Monteith • Use a reference crop to which which water use of other crops are related • Definition A hypothetical crop with an assumed height of 0.12 m, having a surface resistance of 70 s m-1 and an albedo of 0.23, closely resembling the evaporation of an extensive surface of green grass on uniform height, actively growing and adequately watered.

30. Relating crop water requirement to reference

31. Relating crop water requirement to reference

32. Relating crop water requirement to reference

33. Relating crop water requirement (ETc) to reference evapotranspiration (ET0)

34. Relating crop water requirement (ETc) to reference evapotranspiration (ET0) • ET0 : reference evapo- transpiration • ETc: crop evapo- transpiration • Kc: crop coefficient = relation between crop water use and reference water use

35. CROP COEFFICIENT APPROACH single crop coefficient used for applications to irrigation planning, design, and management. used for calculating crop evapotranspiration under standard conditions, ETc. standard conditions refer to crops grown in large fields under excellent agronomic soil water conditions. crop evapotranspiration ETc different from reference evapotranspiration, ETo, ground cover, canopy properties & aerodynamic resistance of the crop are different from grass. effects of characteristics that distinguish field crops from grass are integrated into the crop coefficient, Kc.

36. Effect of weather On evapotranspiration in ET0 Effects of cropped surface as differ from reference surface are integrated into the crop coefficient, ETc = Kc.ET0 Effects of soil water stress Ks multiplying Kc.Ks ETc = Kc.Ks.ET0

37. ETc = Kc.ET0 ETc is the crop evapotranspiration [mm d-1]Kc is the crop coefficient [dimensionless]ET0 is the reference evapotranspiration [mm d-1] most weather effects in ET0 estimate. ET0 = index of climatic demand, Kc varies specific crop characteristics only to a limited extent with climate. transfer Kc values between locations & climates reason for global acceptance & usefulness Kc Kc factors developed in past studies

38. 4 main differences: ET0 vs ETc Crop height influences the aerodynamic resistance term in the FAO Penman-Monteith equation turbulent transfer of vapour from crop into atmosphere. Albedo (reflectance) of the crop-soil surface affected by the fraction of ground covered by vegetation by soil surface wetness. influences net radiation of surface, Rn, primary source of energy for evaporation process. Canopy resistance of crop to vapour transfer affected by leaf area index (number of stomata), leaf age & condition, & degree of stomatal control. influences surface resistance. Evaporation from soil, especially exposed soil.

39. KcpredictsETcunder standard conditions • No limitations are placed on • crop growth; • evapotranspiration due to water shortage; • crop density; • disease, weed, insect or osmotic potentials. • ETc predicted by Kc can be adjusted if necessary to non-standard conditions, • where any environmental condition or characteristic is known to have an impact on or to limit ETc. • Factors for correcting ETc to an adjusted value are described in Allen et al. (1998) and section 10.6.

40. 1. CROP TYPE Differences in albedo, crop height, aerodynamic properties, & leaf and stomata properties, =>ET from full grown, well-watered crops differs from ET0. Crop characteristics influence on Kc: Taller: Kc up Shorter: Kc down Leafier: Kc up Stomatal control: Kc down Closer spacing: Kc up Crops that grow taller with high roughness of full grown crops => have Kc factors larger than 1. 15-20% greater for some tall crops such as maize, sorghum or sugar cane FACTORS DETERMINING Kc

42. 1. CROP TYPE cont FACTORS DETERMINING Kc

43. FACTORS DETERMINING Kc 2. CLIMATE More arid climates & conditions of greater wind speed => Kc up. More humid climates and conditions of lower wind speed => Kc down. Impact of climate on Kc for full grown crops upper bounds = extremely arid and windy conditions lower bounds = very humid & calm weather conditions. Ranges of Kc as climate and weather conditions change small for short crops BUT large for tall crops.

44. FACTORS DETERMINING Kc • 2. CLIMATE

45. FACTORS DETERMINING Kc 3. SOIL EVAPORATION ET= E + T Soil evaporation & crop transpiration in field crops integrated in Kc After rainfall or irrigation, effect of evaporation is dominant when crop is small & little shade on ground For low-cover conditions, Kc coefficient a/c by frequency of soil surface wetting. Where soil is wet for most of time from irrigation or rain, Kc≥ 1. Where soil surface is dry, evaporation is restricted and Kc small as low as 0.1

47. FACTORS DETERMINING Kc 4. CROP GROWTH STAGES As crop develops, ground cover, crop height & leaf area change. Due to differences in ET in various growth stages Kc vary over growing period. Growing period divided - 4 distinct growth stages: initial crop development mid-season late season

48. MAIZE GROWTH STAGES

49. PROCEDURE TO GET CROP COEFFICIENT Kc ID crop growth stages determine lengths of each stage select Kc coefficients adjust Kc values for frequency of wetting or climatic conditions construct crop coefficient curve calculate ETc = ET0 x Kc

50. DEFENITION OF GROWTH STAGES Initial stage: sowing to 10% foliage cover Development stage: 10% to 80% foliage cover Mid-season stage: 80% foliage cover to first signs of senescence Late season: First signs of senescence to physiologically inactive