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abstract 595

DEPENDENCE OF TOTAL EVAPORATION OF IRRIGATED CULTURES ON CLIMATIC FACTORS: THE HYSTERESIS’ EFFECT

Author(s): Yusup Rysbekov

Keyword(s): Water-balance accounts, Evapotranspiration, Climatic factors, Hysteresis’ effect

Article:

Poster:

Session: Poster session 2

AbstractWater-balance accounts (WBA) are a basis of hydrological processes’ understanding, as a whole, and one

of major practical aspects of irrigated lands’ hydrology, in particular. In theory of WBA the wide application was

found the empirical equations, which describe dependence of total evaporation (E) from climatic factors (air

temperature and humidity, solar radiation, etc.). As a rule, in the theory it is considered established that the

dependence of E from climatic parameters is proportional (linear or quasi-linear), and practice of WBA operates

with concept of maximal possible evaporation (Eo), which is accepted for a basis of WBA, and the actual E is

calculated as a share from Eo. In most cases evaporation from water surface is accepted as Eo. Now concept of Eo

has changed, and it is defined as “maximal possible evaporation at optimum soil moisture”. But a new concept of Eo

is closely adhered to the climatic factors also and is “a water equivalent of heat resources”, i.e. again total

evaporation E (evapotranspiration) is put in direct dependence on the climatic factors (“heat resources”).

Therefore in majority of cases the methods of definition of plants needs to water are considered as dependence

of the type
E = f (Emax), where
Å is function of Eo (Emax) for the certain time period.
For example, in

Central Asia for WBA the dependence
E = Emax in a degree 1.58/31.62 is applied, where
Å is

evapotranspiration,
Emax is water evaporation according to the formula by Ivanov-Molchanov:
Emax =

0.00144 (Ò + 25)(Ò + 25)(100 - H), where
Ò is monthly average temperature, and
H is relative air humidity.

At the same time, the long-term researches, which have been carried out on largest lysimeters of former Union of

SSR (area everyone is equaled 25 m2, culture - cotton, Tashkent oasis, Uzbekistan) have shown that the linear

dependence E = f (Emax), is a special case only. In the general case this dependence is nonlinear, and for the

separate periods – opposite. So, where connection E = f (Emax) is used for the monthly data, we will receive a

hysteresis’ loop, and a curve of dependence E = f (Emax) for the period April -October is necessary to divide into

two curves - for the periods April-July and August - October, as they have various character. Accordingly, use of

common curve of dependence E = f (Emax) for WBA will result in overestimate of evapotranspiration’s volume for

the period April - July and in reduction of its volume for the period August - October. For the separate summer

periods a paradoxical connection between E and Emax is observed, namely, Emax (evaporation from water surface)

is reduced, but Å (evapotranspiration from cotton’s field) continues to be increased. Given fact is not taken into

account in the theory of WBA. Account of the hysteresis effect at accounts of water consumption by plants (delay in

time of maximum of E concerning Emax) considerably raises accuracy of WBA for irrigated lands.

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DEPENDENCE OF TOTAL EVAPORATION OF IRRIGATED CULTURES ON CLIMATIC FACTORS: THE HYSTERESIS’ EFFECT