Irrigation Water Calculation Standard

• This Philippine National Standard provides the guidelines and requirements for irrigation water calculations to meet performance standards.

References

• This document references the following normative documents.
• Provisions from the following documents constitute this National Standard when referenced.
  • ASTM D 422: Standard Test Method for Particle-Size Analysis of Soils
  • PNS/BAFS/PAES 219:2017: Performance Evaluation of Open Channels – Determination of Seepage and Percolation by Ponding Method
  • PNS/BAFS/PAES 220:2017: Performance Evaluation of Open Channels – Determination of Seepage and Percolation by Inflow-Outflow Method

Definitions

• Several terms apply to this standard:
  • Actual crop evapotranspiration (ETa): rate of water evaporation from soil and transpiration from plants; equals or is less than the predicted ETcrop, factoring available soil water, salinity, field size, or other causes.
  • Application efficiency (Ea): the ratio of average irrigation water depth infiltrated and stored in the root zone, relative to the average depth of water applied.
  • Conveyance efficiency (Ec): the inlet-to-headwork ratio of water received for a block of fields to that released.
  • Crop coefficient (kc): the ratio of actual crop evapotranspiration to reference evapotranspiration.
  • Crop evapotranspiration: evapotranspiration rate for disease-free crops, grown in fields (1+ ha) under optimal conditions like water and fertilizer, achieving full production potential, including water loss via transpiration and evaporation from soil and wet leaves.
  • Cropping pattern: the sequence of different crops grown methodically on a field.
  • Crop water requirement (CWR): the evapotranspiration and consumptive use, including seepage and percolation losses (amount of water used in producing crops).
  • Diversion water requirement (DWR): the total water volume diverted from streams, lakes, or reservoirs or removed from the ground to irrigate a crop.
  • Effective rainfall (ER): rainfall amount directly on the field and used for crop growth and development, subtracting deep percolation, runoff and interception.
  • Effective rooting depth: soil depth from which the bulk of the crop roots extracts most of the water for evapotranspiration
  • Evapotranspiration: water combination transpired from plants and evaporated from soil, water, and plant surfaces.
  • Farm water requirement (FWR): water amount needed to replenish the crop water and losses, minus the effective rainfall.
  • Hydrologic frequency analysis: chance estimation/likelihood of a given event occurrence, determining best-fit frequency curves to hydrologic data samples.
  • Land preparation water requirement (LPWR): water needed in lowland rice production, covering evaporation, seepage, percolation, and land soaking losses.
  • Land soaking water requirement (LSR): water is required in lowland rice production, dependent on the soil's initial moisture and properties.
  • Pan coefficient: ratio between reference evapotranspiration (ET॰) and evaporation from an open pan.
  • Pan evaporation: water loss rate by evaporation from open pan surface water.
  • Percolation: downward water flow below the root zone, impacted and affects soil structure, texture, bulk density, mineralogy, organic matter, salt/concentration, and composition.
  • Reference crop evapotranspiration (ET。): rate of evaporation from reference surface (hypothetical reference crop, assumed 0.2m height, a fixed 70 s/m surface resistance, 0.23 albedo).
  • Residual moisture content: soil moisture before initial irrigation water delivery, defines water depletion from the soil when the water supply is cut-off.
  • Seepage: water escape, out or below from facilities (open ditches, canals, waterways).

Initial Investigation and General Procedures for calculating irrigation water requirements

• Sufficient information should be known prior to determining irrigation water requirements; Table 1 lists such data:
  • Area to be irrigated: location, layout/shape, fixed boundaries/obstructions and a copy of the area map obtained; details on elevation/topography.
  • Water supply: physical/chemical suitability, location, reliability of water supply.
  • Soil: physical characteristics, risk to erosion.
  • Crop: type/suitability to climate and soil assessed.
  • Meteorological Data: Rainfall /evapotranspiration records availability, prevailing wind direction/speed.
  • Irrigation Method: Future irrigation/flexibility.
• A layout of irrigation components & procedures for determining water requirements in shown in (Figure 1 and Table 2, respectively).

General Procedures for Determining Irrigation Water Requirements

• Type of crop is relevant for planning the cropping calendar and crop coefficient.
• Evapotranspiration and other meteorological data is relevant for determination of actual Evapotranspiration.
  • Lowland CROP WATER REQUIREMENT = ETa + (S&P)field
  • Upland CROP WATER REQUIREMENT = ETa + (S&P)field
• Type of soil is relevant for determination of seepage and percolation rate in the field (S & P)field.
• Depth of root zone and soil physical properties are relevant for FARM WATER REQUIREMENT.
• Residual moisture content & service area are relevant for the determination of land soaking and land preparation water requirement(LPWR).
• Required standing water & service area are relevant for determination of effective rainfall (ER).
  • FWR = CWR - ER + LPWR + farm ditch losses
  • FWR = CWR - ER + farm ditch losses
  • FWR = (CWR-ER+LPWR)/Ea or FWR = (CWR-ER)/Ea
• Type of soil is relevant for determination of farm ditch losses.
• Wetted perimeter and length of farm ditches are relevant for determination of farm ditch losses.
• Type of field application system & determination of application efficiency.
• Canal lining properties & wetted perimeter/length of canals (sub-lateral, lateral, main) are relevant for CONVEYANCE LOSSES.
  • DWR = FWR + conveyance losses or DWR= FWR/Ec
• Leakage through gates and leakage through canal dikes are relevant for determination of conveyance losses.

Development of Cropping Calendar

• Crop type is selected according to climate to maximize elements of nature.
  • Climate/crop descriptions, climate-ideal crops, planting months/growing periods are presented in (Table A.1 to Table A.4 of Annex A).
• A cropping pattern is selected to allow increased production; Patterns include:
  • Crop Rotation: crops are planted yearly.
    • EX) Rice-Soybean-Rice.
  • Multiple Cropping: crops(>3) grow in a sequence, after one another.
    • EX) Rice-Mungbean-Rice-Pechay.
  • Intercropping: duration catch crops are planted between rows of duration crops.
    • EX) Coconut-Rice.
  • Relay cropping: where different are planted in a tight schedule.
    • EX) Rice-Soybean (planted before harvest of rice)-Sweet potato-Mungbean.
• Both dry and wet seasons are delineated to determine when to start cropping and prevent water availability risk periods.
• Yearly farming activities and relative durations are identified; The cropping calendar is planned on a decadal or weekly basis.
  • Cropping calendars for various crops are presented in (Figures A.1 to A.4 of Annex A).
  • An example of a complete cropping calendar is shown inTable 3.

Crop Water Requirement

• It is calculated using Annex H.1.

Actual Evapotranspiration

• Actual evapotranspiration is calculated using Annex H.2.
• ET. must be computed from available meteorological data, according to method of selection and details of computation in Annex B.
• Table 4 provides kc values obtained from experimental plots, ke varies by growth stage and must be reflected in the cropping calendar.

Seepage and percolation in the field/service area

• Use Table 5 reference values to determine seepage/percolation.
• Annex C procedures are referenced if soil texture information is unavailable.

Farm Water Requirement

• Annex H.3 specifies the calculations for farm water requirement.

Effective rainfall

• A minimum of 10-year rainfall data is used for determining effective rainfall.
• It is calculated via hydrologic frequency analysis, as detailed in Annex D or the Asian Development Bank (ADB) method (detailed in Annex E).

Land soaking requirement

• Crop, root zone depth, and soil properties are be obtained with Annex C specifying soil textures.
• Land soaking requirements computed via Annex H.4, calculated in Annex F.

Land preparation water requirement

• Land preparation water is the sum of land soaking requirements, standing water, and replenishment for evaporation; Land preparation water requirement calculated in Annex H.5.
• The recommended standing water value in land preparation is 10 mm.

Farm ditch losses

• Seepage and percolation in ditches utilizes reference value, and canal/ponding/inflow-outflow method dimensions; can only be applied to existing irrigation (PNS/BAFS/PAES 219:2017 & 220:2017).
• Farm ditch losses calculated via Annex H.6.

Application efficiency

• Application losses are expressed using field application efficiency based on a field application system, corresponding efficiency values specified in (Table 6).

Diversion Water Requirement

• Annex H.7 details calculation for diversion water requirement.

Conveyance losses

• These losses in distribution structures, are: supplementary/main farm ditches, lateral/sub-lateral/main canals determined to account for losses, (Annex G).
• Values for planning conveyance (Ec) are in (Table 7), based on canal lining and the technical/managerial facilities of water control.