PNS 217:2017: Irrigation Water Requirements

• Philippine National Standard PNS/BAFS/PAES 217:2017's topic is the determination of irrigation water requirements.
• The Agricultural Machinery Testing and Evaluation Center (AMTEC) initiated the formulation of this national standard.
• Republic Act 10601, also known as the Agricultural and Fisheries Mechanization Law (AFMech Law of 2013), mandates BAFS to develop standard specifications and test procedures.

Scope:

• This standard provides the guidelines and minimum requirements in calculating irrigation water requirements to meet required performance standards.

Definitions:

• Actual Crop Evapotranspiration (ETa): Rate of evapotranspiration equal to or smaller than predicted ETcrop as affected by available soil water, salinity, or field size.
• Application Efficiency (Ea): Ratio of average depth of irrigation water infiltrated and stored in the root zone to the average depth of irrigation water applied.
• Conveyance Efficiency (Ec): Ratio between water received at the inlet for a block of fields to that released at the project's headwork.
• Crop Coefficient (kc): Ratio of the actual crop evapotranspiration to its potential evapotranspiration.
• Crop Evapotranspiration: Rate of evapotranspiration of a disease-free crop growing in a large field under optimal conditions, including sufficient water and fertilizer.
• It achieves full production potential and includes water loss through transpiration and vaporation.
• Cropping Pattern: Sequence of different crops grown in regular order on any particular field or fields.
• Crop Water Requirement (CWR): Amount of water used in producing crops, which is the sum of evapotranspiration or consumptive use plus seepage and percolation losses.
• Diversion Water Requirement (DWR): Total quantity of water diverted from a stream, lake, or reservoir, or removed from the ground to irrigate a crop.
• Effective Rainfall (ER): Amount of rainwater that falls directly on the field that is used by the crop for growth and development, excluding deep percolation, surface runoff, and interception.
• Effective Rooting Depth: Soil depth from which the bulk of the roots of the crop extracts most of the water needed for evapotranspiration.
• Evapotranspiration: Combination of water transpired from vegetation and evaporated from the soil, water, and plant surfaces.
• Farm Water Requirement (FWR): Amount of water to replenish the crop water requirement and losses less the effective rainfall.
• Hydrologic Frequency Analysis: Estimation of the chance or likelihood of occurrence of a given event by determining the frequency curves of best fit to samples of hydrologic data
• Land Preparation Water Requirement (LPWR): Amount of water required in lowland rice production, including water losses through evaporation, seepage, percolation, and land soaking.
• Land Soaking Water Requirement (LSR): Amount of water required in lowland rice production that is a function of the initial soil moisture and the physical properties of the soil.
• Pan Coefficient: Ratio between reference evapotranspiration (ET॰) and water loss by evaporation from an open water surface of a pan.
• Pan Evaporation: Rate of water loss by evaporation from an open water surface of a pan.
• Percolation: Vertical flow of water below the root zone affected by soil structure, texture, bulk density, mineralogy, organic matter content, salt type, and concentration.
• Reference Crop Evapotranspiration (ET。): Rate of evapotranspiration from a reference surface with an assumed crop height of 0.2 m, a fixed surface resistance of 70 s/m, and an albedo of 0.23.
• Residual Moisture Content: Moisture left in the soil before the initial irrigation water delivery, which describes the extent of water depletion from the soil when the water supply has been cut off.
• Seepage: Water escaping below or out from water conveyance facilities such as open ditches, canals, natural channels, and waterways.

Initial Investigation and General Procedures:

• Sufficient information must be known prior to determining irrigation water requirements.
• A layout of irrigation components and the steps in determining irrigation water requirements are needed.
• Details on location, layout/shape, fixed boundaries and obstructions, and a copy of the area map are required
• Physical and chemical suitability, location, and supply reliability of water supply is required
• Soil physical characteristics and risk to erosion must be determined.
• The type of crop/s and suitability to climate and soil must be assessed.
• Availability of rainfall and evapotranspiration records near the area is to be determined, including wind direction and speed
• Identify type of irrigation method and future flexibility

General Procedures:

• Type of crop is relevant for planning the cropping calendar and estimating the crop coefficient.
• Evapotranspiration data is used to determine actual evapotranspiration (ETa).
• Soil type helps determine the seepage and percolation rate in the field (S & P)field.
• Depth of root zone and soil physical properties are important for estimating land soaking and land preparation water requirement (LPWR).
• Rainfall data is needed to calculate effective rainfall (ER).
• Wetted perimeter and length of farm ditches help determine farm ditch losses.
• The type of field application system is relevant for determining application efficiency.
• Canal lining properties, wetted perimeter and length of canals, and potential leakage inform determination of conveyance losses.
• CWR = ETa + (S&P)field
• For upland, CWR = ETa + (S&P)field.
• Using FWR(Farm Water Requirement), FWR = CWR - ER + LPWR + farm ditch losses
• Or FWR = (CWR-ER+LPWR)/Ea
• Diversion Water Requirement, DWR = FWR + conveyance losses or
• DWR = FWR/Ec

Development of Cropping Calendar:

• Select a crop appropriate to the climate to maximize the use of natural elements.
• Climate descriptions, ideal crop types, planting months, and growing periods are detailed in Annex A Tables A.1 to A.4.
• A cropping pattern is selected to allow increased production.
• Strategies include Crop Rotation, Multiple Cropping, Intercropping, and Relay Cropping.
• Delineate dry and wet seasons using simultaneous plots of rainfall and evapotranspiration to determine ideal cropping start times, while avoiding water availability risks.
• Farming activities and duration are identified, with calendars planned on decadal or weekly bases.
• Projected cropping calendars for crops are in Figures A.1 to A.4 of Annex A.

Crop Water Requirement:

• Crop water requirement is calculated using Annex H.1.
• Actual Evapotranspiration, is calculated using Annex H.2 and computed using available meteorological data.
• Values of kc from experimental plots are in Table 4, noting that it varies by growth stage and is reflected in the cropping calendar.

Seepage and Percolation:

• Determined by reference with values listed in Table 5..
• If soil texture is unavailable, use procedures listed in Annex C.

Farm Water Requirement:

• The farm water requirement is calculated using Annex H.3.

Effective rainfall:

• Determined using a minimum of 10 years of rainfall data; hydrologic frequency analysis (Annex D) or the Asian Development Bank (ADB) method (Annex E) can be used.

Land Soaking:

• Information gathered on crop depth and soil physical properites can be based on the soil texture (Annex C).
• Land soaking requirement will be using Annex H.4.
• Details of calculation are presented in Annex F.

Land preparation is defined as:

• Total land soaking water requirement, standing water and replenishment for evaporation.
• Land preparation figures are calculated with the calculation using Annex H.5.
• NOTE The recommended value for standing water during land preparation is 10 mm.

Farm Ditch Losses:

• Seepage and percolation farm losses are determined by using reference values of seepage and percolation and canal dimensions or ponding method and inflow-outflow method.
• These methods can only be used in existing irrigation systems.
• Farm ditch losses are calculated using Annex H.6.

Application efficiency:

• Determined using values of field application efficiency based on the type of field application system.
• Table 6 shows the values of application efficiency.

Diversion Water Requirement:

• Calculated using Annex H.7.
• Conveyance losses are seepage and percolation in conveyance structures. For example, supplementary farm ditches, main farm ditches, lateral canals, sub-lateral canals and main canal are determined to account for the conveyance losses, using teh process of computation is presented in Annex G.
• Reference figures for conveyance efficiency (Ec) can be used for planning. Table 7 has conveyance efficiency depending on canal lining, control facilities, and management

Annex B:

• Specifies in guidelines for calculating the reference crop evapotranspiration using available meteorological data and the selection of type of calculation.
• Details duration and accuracy of four methodes: Penman-Montieth, Radiation, Blaney-Criddle and Pan Evaporation. -Details data requirments for each method climatic conditions required.
• Provides equations and formulas for calculating evapotranspiration.

This standard specifies guidelines and minimum requirements for calculating irrigation water needs, aligning with Republic Act 10601. It defines key terms like Actual Crop Evapotranspiration (ETa) and Application Efficiency (Ea) for effective water management. The standard aims to meet performance standards in irrigation practices.