Cropwat8.0Exampl

EXAMPLE OF THE USE OF CROPWAT 8.0

1

1.

INTRODUCTION

This document shows in a practical way the use of CROPWAT 8.0 for design and
management of irrigation schemes, taking the user, with the help of an actual data
set, through the different steps required to calculate evapotranspiration, crop water
requirements, scheme water supply and irrigation scheduling.
To learn about how thesoftware works and the main calculation procedures, users
are invited to read the context-specific Help available in the software.
The example set used is taken from the Rajolibanda Diversion Scheme in Andhra
Pradesh, India.

2

2.

CALCULATION OF REFERENCE EVAPOTRANSPIRATION

2.1

Introduction

The Reference Evapotranspiration (ETo) represents the potential evaporation of awell-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 as the appropriate combination method1 to determine ETo from
climatic data on:
•
•
•
•

2.2

temperature
humidity
sunshine
windspeed.

Climate Data Collection

In order to calculateETo, 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 beconsidered for crop water requirement (CWR) calculations.
In some cases, when the scheme is large, more than one station may be available,
but often no suitable stations with sufficient climatic data are located in the scheme.
In such a case a careful selection should be made of the data.
In our example, the data for the Rajolibanda Scheme have been obtained from the
following sources:Temperature:
Humidity:
Wind:
Sunshine hours:

2.3

IMS Station, Kurnool (1930-1960)
IMS Station, Kurnool
IMS Station, Kurnool
Airport, Hyderabad

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, attentionshould be given to the units in
which the climatic records are given.
As an example, the following notes for conversion of the climatic data provided by
the IMS to the units required for CROPWAT 8.0 are given.
1

For further reference see FAO Irrigation and Drainage Paper No. 56, 1998. Crop Evapotranspiration – guidelines for
computing crop water requirements,.

3

Temperature data
IMS:CROPWAT:
Conversion:

Mean Daily Maximum and Minimum Temperature, [º C]
Maximum and Minimum Temperature, [º C]
No conversion needed

Humidity data
IMS:
CROPWAT:
Conversion:
Comments:

Relative Humidity [in percentage] as well as the Vapour Pressure
[kPa] both for morning and afternoon
Average daily Relative Humidity [in percentage] or Vapour Pressure
[kPa]
Average of morning andafternoon values of vapour pressure
Vapour pressure rather than Relative Humidity values are taken, as
the latter relate to temperature values at sunrise and noon

Sunshine data
IMS:
CROPWAT:
Conversion:

Cloudiness in Oktas of sky of All and Low Clouds for morning and
noon
Sunshine hours (heliograph) or sunshine percentage
According to the following relationship:

SSP = 0.95 where:SSP
LC1
LC2
AC1
AC2
α

=
=
=
=
=
=

LC1 + LC2
AC1 + AC2
+α *
2
2
sunshine rate (fraction)
low clouds at sunrise (oktas)
low clouds at noon (oktas)
high clouds at sunrise (oktas)
low clouds at noon (oktas)
empirical parameter (≈ 0.3)

Windspeed data
IMS:
CROPWAT:
Conversion:

2.4

Average Daily Windrun in km/hour
Average Daily Windspeed in km/day or m/sec
WS...