Estimation of irrigation requirement using CROPWAT

December 17

2013

Amit Yalwar Sushma M

1.

(a) Differentiate between Reference Evapotranspiration and Actual Crop Evapotranspiration

Reference Evapotranspiration Actual Crop Evapotranspiration

The evapotranspiration rate from a reference

surface is called the reference crop

evapotranspiration or reference evapotranspiration

and is denoted as ETo. The reference surface is a

hypothetical grass reference crop with specific

characteristics. The assumed characteristics are:

crop height of 0.12 m

fixed surface resistance of 70 s m-1

an albedo of 0.23

Actual evapotranspiration in a soil water budget is

the actual amount of water delivered to the

atmosphere by evaporation and transpiration. In wet

months, when precipitation exceeds potential

evapotranspiration, actual evapotranspiration is equal

to potential evapotranspiration. In dry months, when

potential evapotranspiration exceeds precipitation,

actual evapotranspiration is equal to precipitation

plus the absolute value of the change in soil moisture

storage. Denoted by AET

ETo was introduced to study the evaporative

demand of the atmosphere independently of crop

type, crop development and management

practices. As water is abundantly available at the

reference evapotranspiring surface, soil factors do

not affect ET. The only factors affecting ETo are

climatic parameters. Consequently, ETo is a climatic

parameter and can be computed from weather

data. ETo expresses the evaporating power of the

atmosphere at a specific location and time of the

year. The use of potential ET is strongly

discouraged due to ambiguities in its definition.

The reference crop evapotranspiration represents the

evapotranspiration from a standardized vegetated

surface where as AET represents the real

evapotranspiration occurring in a specific situation.

Actual Evapotranspiration is dependent on several

factors such as Weather parameters, Crop factors,

and Management & environmental conditions.

(b) List some of the important factors affecting crop evapotranspiration

The factors that affect the crop evapotranspiration are:

Weather parameters such as radiation, air temperature, humidity and wind speed

Crop factors such as crop type, crop height, crop roughness, reflection, ground cover and crop rooting

Management and environmental conditions such as cultivation practices, application of fertilizers, availability

of water and the type of irrigation method employed

(c) For the FAO-56 Penman-Monteith method write the following

i) Expression for ETo

Where

Rn is the net radiation,

G is the soil heat flux,

(es - ea) represents the vapour pressure deficit of the air,

ra is the mean air density at constant pressure,

cp is the specific heat of the air,

D represents the slope of the saturation vapour pressure temperature relationship,

g is the psychrometric constant, and

rs and ra are the (bulk) surface and aerodynamic resistances

ii) How to calculate ET from ETo by using the crop coefficients

Crop evaporation is calculated by multiplying the reference crop evapotranspiration, ETo, by a crop coefficient, Kc:

ETc = Kc ETo Where,

ETc crop evapotranspiration [mm d-1],

Kc crop coefficient [dimensionless],

ETo reference crop evapotranspiration [mm d-1]

The reference ETo is defined and calculated using the FAO Penman-Monteith equation.

There are two approaches to determine crop coefficients

i. Single crop coefficient approach (Kc)

In the single crop coefficient approach, the effect of crop transpiration and soil evaporation are

combined into a single Kc coefficient. The coefficient integrates differences in the soil evaporation and

crop transpiration rate between the crop and the grass reference surface. As soil evaporation may

fluctuate daily as a result of rainfall or irrigation, the single crop coefficient expresses only the time-

averaged (multi-day) effects of crop evapotranspiration.

ii. Dual crop coefficient approach (Kcb + Ke)

In the dual crop coefficient approach, the effects of crop transpiration and soil evaporation are

determined separately. Two coefficients are used: the basal crop coefficient (Kcb) to describe plant

transpiration, and the soil water evaporation coefficient (Ke) to describe evaporation from the soil

surface. The single Kc coefficient is replaced by:

Kc = Kcb + Ke Where,

Kcb basal crop coefficient,

Ke soil water evaporation coefficient

2. Estimation of crop evapotranspiration using CROPWAT

(a) Give a short description about CROPWAT, mentioning its purpose/ functions, organization behind its

development/ maintenance.

Purpose/Functions and features: CROPWAT for Windows is a computer program for the calculation of crop water

requirements and irrigation requirements based on soil, climate and crop data. In addition, the program allows the

development of irrigation schedules for different management conditions and the calculation of scheme water supply

for varying crop patterns. CROPWAT can also be used to evaluate farmers irrigation practices and to estimate crop

performance under both rainfed and irrigated conditions. There are many features built in to CROPWAT, some of

which are as follows:

monthly, decade and daily input of climatic data for calculation of reference evapotranspiration (ETo)

backward compatibility to allow use of data from CLIMWAT database

possibility to estimate climatic data in the absence of measured values

decade and daily calculation of crop water requirements based on updated calculation algorithms including

adjustment of crop-coefficient values

calculation of crop water requirements and irrigation schedulingfor paddy & upland rice, using a newly

developed procedure to calculate water requirements including the land preparation period

interactive user adjustable irrigation schedules

daily soil water balance output tables

easy saving and retrieval of sessions and of user-defined irrigation schedules

graphical presentations of input data, crop water requirements and irrigation schedules

easy import/export of data and graphics through clipboard or ASCII text files

extensive printing routines, supporting all windows-based printers

context-sensitive help system

Multilingual interface and help system: English, Spanish, French and Russian

Organization behind its development/maintenance: CROPWAT is a decision support tool developed by the Land

and Water Development Division of FAO. The Land and Water Division aims at enhancing the agricultural

productivity and advancing the sustainable use of land and water resources through their improved tenure,

management, development and conservation. It addresses the challenges member countries face in ensuring

productive and efficient use of land and water resources in order to meet present and future demands for agricultural

products, while ensuring the long-term sustainability of the land and water quantity and quality.

FAO is engaged in a programmatic approach to agricultural water management that addresses water use efficiency

and productivity, and deploys best practices for water use and conservation. The Division further develops and

maintains various information systems, including its continually updated water information system, AQUASTAT, and

tools for analysis such as CROPWAT, AQUACROP and MASSCOTE, thereby contributing to the formulation of

national and regional water management strategies and perspective studies.

(b) List various modules available in CROPWAT and briefly mention the functions of each of the

modules.

The modules available in CROPWAT are as follows:

Climate/ETo

The Climate module can be selected by clicking on the Climate/ETo icon in the module bar located on the of

the main CROPWAT window. The module is primary for data input, requiring information on the meteorological

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

Rain

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.

Crop

The Crop module requires crop data over the different development stages, defined

as follows:

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

ii. Development stage: it runs from 10% ground cover to effective full cover.

iii. Effective full cover for many crops occurs at the initiation of flowering.

iv. 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.

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

Soil

The module includes calculations, providing the initial available soil moisture and, in case of rice, the maximum

percolation rate after puddling.

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

i. Total Available Water (TAW)

ii. Maximum infiltration rate

iii. Maximum rooting depth

iv. Initial soil moisture depletion

CWR(Crop Water Requirement)

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.

Schedule

i. The schedule module essentially includes calculations, producing a soil water balance on a daily step. The

following parameters are used:

ii. Effective rainfall, over dry, normal and/or wet years

iii. Water stress coefficient (Ks)

iv. Crop evapotranspiration under non-standard conditions (ETc adj)

v. Root zone depletion

vi. Net irrigation

vii. Deficit

viii. Irrigation losses

ix. Gross irrigation

x. Flow

Crop Pattern

The cropping pattern module is primary data input, requiring information on the crops (Up to 20) being part of

the scheme. With reference to each crop, the following data should be provided:

i. Crop file

ii. Planting date

iii. Area: extension of the area dedicated to each crop, as a percentage of the total cropped area. Care

should be taken that at any given moment the sum of the individual crops does not exceed 100 % of total

scheme area.

Scheme

The scheme module includes calculations, producing:

i. Irrigation requirement for each crop of the scheme

ii. Net scheme irrigation requirement

iii. Irrigated area as a percentage of the total area

iv. Irrigation requirement for the actual area

(c) download the meteorological data for

- Kurnool district in Andhra Pradesh

- Get the annual average of

Maximum temperature

Minimum temperature and

Rainfall

Get the latitude/longitude and altitude of Kurnool from any other sources

a. Annual average Maximum temperature from 1901 to 2002

b. Annual average Minimum Temperature from 1901 to 2002

Month Temp oC

Jan 17.12696

Feb 19.11847

Mar 22.00961

Apr 25.04699

May 25.5075

Jun 24.33742

Jul 23.35353

Aug 23.05736

Sep 22.61122

Oct 21.71819

Nov 19.16203

Dec 16.98119

Month Temp oC

Jan 30.30075

Feb 33.26194

Mar 36.84572

Apr 39.02913

May 38.80286

Jun 34.63322

Jul 31.95679

Aug 31.49069

Sep 31.76408

Oct 31.32451

Nov 29.70408

Dec 28.89462

c. Annual Average Rainfall from 1901 to 2002

Month mm

Jan 1.092598

Feb 1.279314

Mar 3.59452

Apr 17.79497

May 46.0072

Jun 50.45806

Jul 60.67507

Aug 71.283

Sep 132.8518

Oct 108.6945

Nov 40.31686

Dec 8.175127

d. Latitude: 15o50N

Longitude: 78o05E

Altitude: 274-281m above MSL (Various sources quote values in between 274m and 281m)

(d) Estimate the irrigation requirement for a selected crop and soil. Calculate the total irrigation

requirement during different growth periods of the crop (initial stage, crop development stage etc.)

Crop chosen: Rice

Soil type chosen: Black Clay Soil

Total Irrigation requirement: 917.1 mm/dec

Stage-wise Irrigation requirement:

Month Decade Stage Kc ETc ETc Eff rain Irr. Req.

coeff mm/day mm/dec mm/dec mm/dec

Nov 1 Nursery 1.2 0.46 0.5 1.8 0.5

Nov 2 Nurs/LPr 1.19 0.79 7.9 11.5 90.4

Nov 3 Nurs/LPr 1.06 3.85 38.5 8.6 30

Dec 1 Initial 1.07 3.81 38.1 5.3 178.6

Dec 2 Initial 1.1 3.86 38.6 1.6 37

Dec 3 Development 1.1 3.98 43.8 1.2 42.6

Jan 1 Development 1.13 4.19 41.9 0.9 41

Jan 2 Development 1.16 4.44 44.4 0.1 44.4

Jan 3 Mid 1.2 4.89 53.8 0.2 53.6

Feb 1 Mid 1.21 5.24 52.4 0.4 52

Feb 2 Mid 1.21 5.56 55.6 0.4 55.2

Feb 3 Mid 1.21 5.96 47.7 0.7 47

Mar 1 Late 1.2 6.36 63.6 0.6 63

Mar 2 Late 1.17 6.59 65.9 0.7 65.2

Mar 3 Late 1.12 6.5 71.5 2.4 69.1

Apr 1 Late 1.07 6.43 51.4 3.1 47.5

Total 715.6 39.4 917.1