CHAPTER 6: Irrigation scheduling

1 CHAPTER 6: Irrigation schedulingIrrigation scheduling is one of the factors that influence the agronomicand economic viability of small farms. It is important for both watersavings and improved crop yields. The Irrigation water is applied to thecultivation according to predetermined schedules based upon themonitoring of: the soil water status; the crop water type of soil and climatic conditions have a significant effect onthe main practical aspects of Irrigation , which are the determination ofhow much water should be applied and when it should be applied to agiven addition to the basic factors relevant to the preparation of irrigationschedules examined below, other important elements should also beconsidered, such as crop tolerance and sensitivity to water deficit atvarious growth stages, and optimum water RELATIONSHIPT able presents a summary table of soil physical Irrigation TechniquesPressurized Irrigation of soilSaturation capacity (SC)

2 % weightField capacity (FC) % weightWilting point (WP) % weightSC/FCFC/WPBulk density(volume weight)Soil available water (moisture) by volume(FC-WP xbulk density)Available moisture (Sa) in mm per metre soildepth (FC-WP xbulk density x10)Soil water tension in bars: at field capacity at wilting pointTime required from saturation to field capacityInfiltration rateHeavy (fine)texture55 65%32 42%20 24%2 g/cm316 20%160 200 89 h2 6 mm/hMediumtexture35 45%18 26%10 14%2 g/cm312%120 36 h8 16 mm/hLight(coarse) texture25 35%8 10%4 5%2/12 g/cm36%60 24 h25 75 mm/hTABLE - Soil physical properties (average values)Example:The field capacity (FC) of a 45-cm layer of soil is 18 percent.

3 Howmuch water in cubic metres per hectare does this layer hold?Answer:FC = 18 %, WP = FC = %, Sa = = %;Bulk density = g/cm3; Sa mm/m = x10 = , Sa mm/45cm = x10 = mm; m3/ha = 1 x10 000(1 ha) = , or m3/ha = Sa (mm/m) xdepth of layer (m) , the answer is m3 ROOT DEPTHThis is the soil depth from which the plants take nearly 80 percent oftheir water needs, mostly from the upper part where the root system isdenser. The rooting depths depend on the plant physiology, the type of soil,and the water availability (kind of Irrigation ). Indicative figures arepresented in FAO Irrigation and Drainage Paper No.

4 24, Table general, vegetables (beans, tomatoes, potatoes, onions, peanuts,cucumbers, etc.) are shallow rooted, about 50 60 cm; fruit trees, cottonand some other plants have medium root depths, 80 120 cm. Alfalfa,sorghum, and maize have deeper roots (Table ). Moreover, rootingdepths vary according to DEFICIT OR DEPLETION OF SOIL AVAILABLE water The fraction of moisture in the soil which amounts to 20 70 percent ofthe total available moisture (Sa) and is easily absorbed by the plants(without any stress that results in yield reduction) is called readily availablemoisture. It is a product of Sa multiplied by p, which represents themaximum permissible depletion of available water (moisture).

5 The p valuediffers according to the kind of plant, the root depth, the climaticconditions and the Irrigation techniques. Values for p are given in FAOI rrigation and Drainage Paper No. 33, Tables 19 and 20, and vary fromPressurized Irrigation TechniquesChapter 6 Irrigation - Example of rooting depth (metres) during the growing in shallow rooted sensitive crops to in deep rooted tolerantcrops. Table 23 of the same paper provides information on the sensitivegrowth periods of different observations have shown that the lower the soil moisturedepletion (p), the better the crop development and yield. Hence, therecommended p values are: for shallow rooted seasonal crops; for deep rooted field crops and mature Irrigation APPLICATION DEPTH Irrigation takes place when the permissible percentage (p) of availablewater (Sa) is depleted from the root depth, to replenish the depletedwater.

6 Therefore:Net depth of Irrigation dose (d) (mm) = (Sa p) Dwhere Sais the available water in millimetres per metre, pis thepermissible depletion (fraction), and Dis the root depth (m).Example:Where Sa = 99 mm/m, p = , D = m, what is the net irrigationdose (d) in millimetres to replenish the moisture deficit?d = 99 x x = mmCROP water REQUIREMENTSThe amount of water which evaporates from wet soils and plant surfacestogether with the plant transpiration is called evapotranspiration (ET). Itsvalue is largely determined by climate factors, such as solar radiation,temperature, humidity and wind, and by the environment.

7 Out of the totalevapotranspiration, evaporation accounts for about 10 percent and planttranspiration for the remaining 90 percent. Crop water requirementsencompass the total amount of water used in approaches for estimating the evapotranspiration, such asthe radiation, Penman and pan methods, are presented in FAO Irrigationand Drainage Papers Nos. 24 and 33. Reference evapotranspiration (ETo)represents the rate of evapotranspiration of green grass under idealconditions, 8 15 cm tall, with extensive vegetative cover completelyshading the ground. It is expressed as a mean value in millimetres per dayover a period of 10 30 Irrigation TechniquesPressurized Irrigation Irrigation TechniquesChapter 6 Irrigation most practical method for determining ETo is the pan evaporationmethod.

8 This approach combines the effects of temperature, humidity,wind speed and sunshine. The best known pans are the Class Aevaporation pan (circular) and the Colorado sunken pan (square).The evaporation from the pan is very near to the evapotranspirationofgrass that is taken as an index of ETo for calculation purposes. The pandirect readings (Epan) are related to the ETo with the aid of the pancoefficient (kpan), which depends on the type of pan, its location(surroundings with or without ground cover vegetation) and the climate(humidity and wind speed) (Table ). Hence, ETo = Epan kpan values for both types of pans are given in FAO Irrigation andDrainage Paper No.

9 24, Tables 18 and 19. For the Class A pan the averagekpan is and for the Colorado sunken pan it is :In order to relate ETo to crop water requirements (ETc), the specific cropcoefficient (kc) must be determined: ETc = ETo crop coefficient (kc) depends on the crop leaf area and itsroughness, the stage of growth, the growing season and the prevailingweather conditions (Table ). Tables and list the kc values fordifferent crops at various growth :Total net water requirement approximately 580 mm(December taken as half)TABLE - Estimate of ETo in millimetres per day in the Wadi Tuban ET ET cmm/month7812019213599 TABLE - Cotton, growing season August-DecemberEFFECTIVE RAINFALLIn many areas, seasonal rain precipitation (P) might provide part of thewater requirements during the Irrigation season.

10 The amount of rainwaterretained in the root zone is called effective rainfall (Pe) and should bededucted from the total Irrigation water requirements calculated. It can beroughly estimated as:Pe = P where P > 75 mm/month;Pe = P where P < 75 Irrigation TechniquesPressurized Irrigation developmentMid-seasonLate and harvestBean (green) (dry) (sweet) (grain) (green) (dry) (fresh) - Crop factor (kc) for seasonal crops (average figures)CropYoung , cherry, , apricot, pear, peach, pecan, , palm - Crop factor (kc) for permanent cropsPressurized Irrigation TechniquesChapter 6 Irrigation COVERA nother element to consider when estimating crop water requirementsis the percentage of the field area (ground) covered by the cultivation.