Introduction to CROPWAT software.

UNDERSTANDING OF CROPWAT

Object: To understand the basics of CROPWAT and its various modules.

Accessories: Setup of CROPWAT 8.0 software and initial data.

Theory: There are three fundamental questions of irrigation;

1. How to irrigate?
   
2. How much to irrigate?
   
3. When to irrigate?
   

CROPWAT is a software tool having a decision support system developed by FAO.
Out of the above three question, CROPWAT does not deal with the first question.
But however, it deals with the question of how much to irrigate, and when to irrigate.

Functions of CROPWAT: There are two main functions of CROPWAT:

1. To Calculate:

• • Reference evapotranspiration
    
  • crop water requirements
    
  • crop irrigation requirements.
    

2. To develop:

Irrigation Schedules under various management conditions and scheme of water supply.

PROGRAME STRUCTURE:

• There are total eight modules in the CROPWAT.
  
• Five are the data input modules.
  
• Three are the data output modules or calculated modules.
  

Input Modules:

1. Climate/ Evapotranspiration (ETo)
   
2. Rain
   
3. Crop
   
4. Soil
   
5. Crop pattern
   

Output Modules:

1. CWR (Crop water requirement)
   
2. Schedule
   
3. Scheme
   
4. CLIMATE/ EVAPOTRANSPIRATION (ET_o):
   

The Reference Evapotranspiration (ETo) represents the potential evaporation of a well-watered grass crop. The water needs of other crops are directly linked to this climatic parameter.

Although several methods exist to determine ETo, the Penman-Monteith Method has been recommended by ASCE as the appropriate method to determine ETo from climatic data on:

• temperature

• humidity

• sunshine

• windspeed.

1. Climate Data Collection:
   

In order to calculate ETo, the respective climatic data should be collected from the nearest and most representative meteorological station. Several institutes and agencies may keep climatic records such as the Irrigation Department, the Meteorological Service or nearby Agricultural Research Stations and may provide information on climatic stations inside or in the vicinity of our irrigation scheme which should be considered for crop water requirement (CWR) calculations.

1. Climate Data Conversion:
   

In general, climate data by the National Meteorological Service are standardized.

Normally some conversions are required in order to adjust the data into the format

accepted by CROPWAT 8.0. In particular, attention should be given to the units in

which the climatic records are given.

where:

SSP = sunshine rate (fraction);LC1 = low clouds at sunrise (oktas); LC2 = low clouds at noon (oktas)

AC1 = high clouds at sunrise (oktas); AC2 = low clouds at noon (oktas); α = empirical parameter (» 0.3)

1.3Climate/ETo Data Input and Output:

The Climate module can be selected by clicking on the “Climate/ETo” icon in the module bar located on the left of the main CROPWAT window. The data window will open with the default data type (Monthly / Decade / Daily values); The module is primary for data input, requiring information on the meteorological

station (country, name, altitude, latitude and longitude) together with climatic data.

CROPWAT 8.0 can calculate reference ETo using only temperature, but humidity, wind speed and sunshine should be entered if available. The Climate/ETo module includes calculations, producing Radiation and ETo data using the FAO Penman-Monteith approach.

2. RAINFALL

The rainfall contributes to a greater or lesser extent in satisfying CWR, depending on the location. During the rainy season in tropical and some semi-tropical regions, a great part of the crop’s water needs are covered by rainfall, while during the dry season, the major supply of water should come from irrigation.

How much water is coming from rainfall and how much water should be covered by irrigation is, unfortunately, difficult to predict as rainfall varies greatly from season to season.

In addition to the variability of rainfall, not all rain which falls is used by the crop. Part of the rainfall is lost due to surface runoff or due to deep percolation below the root zone. In order to determine that part of the rainfall which effectively contributes to cover CWR, a number of definitions are first given, while subsequently it is explained how the different rainfall values can be calculated and how they are incorporated in the CWR calculations.

2.1 Rainfall Definitions

Average Monthly Rainfall:

Mathematically determined average for a series of rainfall records, most commonly available. To be used for CWR calculation to represent average climatic conditions.

Dependable Rainfall:

The amount of rainfall which can be depended upon in 1 out of 4 or 5 years corresponding to a 75 or 80% probability of exceedance and representing a dry year. The dependable rainfall (80%) is used for the design of the irrigation system capacity.

Rainfall In Wet, Normal And Dry Years:

Defined as the rainfall with a respectively 20, 50 and 80% probability of exceedance, representing a wet, normal and dry year. The three values are useful for the programming of irrigation supply and simulation of irrigation management conditions. The rainfall in normal years (50% probability) is, in general, well

approached by the average rainfall.

Historical or Actual Rainfall Data:

The actual recorded rainfall used for evaluation purposes.

Effective Rainfall:

Defined as that part of the rainfall which is effectively used by the crop after rainfall losses due to surface run off and deep percolation have been accounted for. The effective rainfall is the rainfall ultimately used to determine the crop irrigation requirements.

2.2 Rainfall Data Collection

The precipitation data required for CROPWAT 8.0 can be daily, decade or monthly rainfall, commonly available from many climatic stations. In addition, substations may be found with single rainfall records. For larger schemes, records of several rainfall stations may be available, allowing an analysis of the spatial variability.

To allow a calculation of rainfall probabilities, rainfall records from a range of years (15-30) are to be collected.

3.3 Rainfall Data Processing

For programming of irrigation water supply and management, rainfall data of normal, wet and dry years are used.

An estimate of the respective rainfall data can be obtained by computing and plotting probabilities from the rainfall records. The different steps involved are:

i. Tabulate yearly rainfall totals for a given period.

ii. Arrange data in descending order of magnitude.

iii. Tabulate plotting position according to:

Fa = 100 * m / (N + 1)

where: N = number of records

m = rank number

Fa = plotting position

iv. Plot values on log-normal scale and obtain the logarithmic regression equation.

v. Calculate year values at 20, 50 and 80% probability.

vi. Determine monthly values for the dry year according to the following relationship:

P_idry = P_iav *

where: Pi av = average monthly rainfall for month i

Pi dry = monthly rainfall dry year for month i

Pav = average yearly rainfall

Pdry = yearly rainfall at 80% probability of exceedance

Similarly values for normal and wet years can be determined.

3.4 Effective Rainfall Method:

To account for the losses due to runoff or percolation, a choice can be made of one of the four methods given in CROPWAT 8.0 (Fixed percentage, Dependable rain, Empirical formula, USDA Soil Conservation Service).

In general, the efficiency of rainfall will decrease with increasing rainfall. For most rainfall values below 100 mm/month, the efficiency will be approximately 80%. Unless more detailed information is available for local conditions, it is suggested to select the Option “Fixed percentage” and give 80% as requested value.

In the water balance calculations included in the irrigation scheduling part of CROPWAT, a possibility exists to evaluate actual Efficiency values for different crops and soil conditions.

3.5 Rainfall Data Input and Output:

The Rain module can be selected by clicking on the “Rain” icon in the module bar located on the left of the main CROPWAT window. The data window will open with the default data type (Daily / Decade / Monthly values).

Once the window is open with the suitable data type, type rainfall data and check the input.

The Rain module also include calculations, producing Effective rainfall data using one of the approaches available, which can be selected by clicking on “Options” on the toolbar while the Rain module is the active window. Rainfall data should be saved after input of one set of data is completed. To do so, select the “Save” button on the Toolbar or the “File” > “Save” menu item.

3.CROP AND CROPING PATTERN:

To determine the irrigation requirements of any Scheme, an assessment should be made of the different crops grown under irrigation presently and possibly in the future. Furthermore, information on the various crop characteristics such as length of the growth cycle, crop factors, rooting depth, etc., should be collected.

CROPWAT 8.0 has crop data for several common crops taken from selected FAO publications. However, the most reliable crop data remain the data obtained from local agricultural research stations.

3.1 Crop and Cropping Pattern Data Collection:

A local survey should be carried out in the irrigation scheme to assess the crops grown rainfed as well as under irrigation. Through field observations, interviews with extension agents and farmers and additional information from other agencies, for instance a revenue department, an assessment can be made of the present cropping pattern.

Essential information collected from the field should include:

1. Crop and crop variety

2. First and last planting date

3. First and last harvesting date

Additional information may include:

4. Indicative yield level

5. Indicative irrigation practices:

• field irrigation methods

• irrigation frequencies and interval

• irrigation application depths

From the Agricultural Research Stations, more accurate information may be collected on:

6. Crop characteristics:

• length of individual growth stages

• crop factors, relating crop evapotranspiration to reference evapotranspiration

• rooting depth

• allowable depletion levels

• yield response factors.

3.1 Crop and Cropping Pattern Data Processing:

The collected information on the planting and harvest dates should be systematically arranged in a cropping pattern. The planting date of crops, in particular those which cover substantial areas or are

high in water demands such as rice, may be spread over a period of 3-6 weeks. In such cases the crop may be subdivided into different crop units with planting date intervals of 10-15 days.

3.2 Crop and Cropping Pattern Data Input and Output:

The Crop module can be selected by clicking on the “Crop” icon in the module bar located on the left of the main CROPWAT window. The data window will open with the default data type (Non-rice / Rice crop); The Crop module requires crop data over the different development stages, defined as follow:

• Initial stage: it starts from planting date to approximately 10% ground cover.

• Development stage: it runs from 10% ground cover to effective full cover. Effective full cover for many crops occurs at the initiation of flowering.

• Mid-season stage: it runs from effective full cover to the start of maturity. The start of maturity is often indicated by the beginning of the ageing, yellowing or senescence of leaves, leaf drop, or the browning of fruit to the degree that the crop evapotranspiration is reduced relative to the ETo.

• Late season stage: it runs from the start of maturity to harvest or full senescence.

Data required differ in case of a non-rice or a rice crop. In case of non-rice crop, the following information is necessary:

• Crop name

• Planting date

• Crop coefficient (Kc)

• Stages length

• Rooting depth

• Critical depletion fraction (p)

• Yield response factor (Ky)

If available, maximum crop height should be provided.

The rice crop module requires the following additional information in comparison to non-rice crops:

• Planting date in case of direct sowing, or transplanting date in case of sowing in a nursery area

• Duration of nursery and land preparation (including puddling) stages

• Dry and Wet Crop coefficients (Kc)

• Puddling depth

4.CROP WATER REQUIREMENT CALCULATIONS:

Calculation of the CWR can be carried out by calling up successively the appropriate climate and rainfall data sets, together with the crop files and the corresponding planting dates.

In case of CWR calculation of rice, soil data are also required.

4.1 Soil Data Collection:

Information from the soil surveys should be obtained and distinct soil characteristics be carried out and different type of soils available and covered areas by each soil type should be determined.

4.2 Soil Data Input and Output:

The Soil module is selected by clicking on the “Soil” icon in the module bar located

on the left of the main CROPWAT window.

The Soil module is essentially data input, requiring the following general soil data:

• Total Available Water (TAW)

• Maximum infiltration rate

• Maximum rooting depth

• Initial soil moisture depletion

In case of rice calculation, the following additional soil data are required:

• Drainable porosity

• Critical depletion for puddle cracking

• Water availability at planting

• Maximum water depth

The module also includes calculations, providing the initial available soil moisture and,

in case of rice, the maximum percolation rate after puddling.

4.3 CWR Data Input and Output:

The CWR module can be selected by clicking on the “CWR” icon in the module bar located on the left of the main CROPWAT window. Data on Climate/ETo, Rainfall (average), Crop and Soil (in case of rice) are required. If not, all data are available, CROPWAT will produce a warning and close the CWR module.

The CWR module includes calculations, producing the irrigation water requirement of the crop on a decadal basis and over the total growing season, as the difference between the crop evapotranspiration under standard conditions (ETc) and the Effective rainfall.

Conclusion:

CROPWAT is a software tool which is very useful for the calculations of crop water requirement (CWR), Crop irrigation requirement and to develop schemes for any irrigation projects, including water requirements of irrigation canals.

This software needs three basic data inputs known as climate or evapotranspiration, rainfall data and crop type and its data to calculate CWR for any type of dry or non-rice crop. However, for rice crop soil data input is also necessary to be given. When schemes for any irrigation projects or irrigation canal projects are to be developed in that case crop patterning is also required as an input in order to calculate the overall scheme of water.