The management of surface water resources such as reservoirs, wetlands
and other freshwater ecosystems needs estimates of open water evaporation.
Also, understanding open water evaporation is necessary in planning economic
uses of these water resources. Open water evaporation serves as a convenient
index of the evaporation demand of a particular climate (Linacre, 2004)
and, plays an important role not only in the water budget of a lake, reservoir
or wetland, but also in the energy budget.
Evaporation and transpiration are major components of the hydrologic
cycle, where approximately 62% of precipitation over land is lost through
these phenomena globally. Several difficulties arise when modeling evaporation
of arid region free water bodies. In these open water bodies, evaporation
is the major component of water balance which generally, has rarely been
measured directly especially in developing countries (Vallet-Coulomb et
al., 2001). Huge amount of sensible heat flows from adjacent warm-hot
dry lands (large amount of advection energy flux density) and increases
the evaporation rate of free surface water bodies drastically. Therefore,
it is very difficult to estimate evaporation of open water using ordinary
Evaporation is estimated by numerous methods such as Penman and Priestley-Taylor
(PT) models (Castellvi et al., 2001; Daneshkar Arasteh, 2004).
Priestley-Taylor formula is one the most used model to estimate open water
evaporation in operational procedures such as reservoir management. It
is presented for no or low advection conditions or on the base of the
equilibrium assumption which is the theoretical lower limit of evaporation
(Jacobes et al., 2002; Li and Yu, 2007). This is the main restriction
of using such a model in arid regions.
Priestley and Taylor coefficient is suggested, αPT =
1.26 for minimum advective condition with no edge effects (Eaton et
al., 2001). Pereira and Nova (1992) emphasized that the value of 1.26
is suitable for potential condition and invalid under advective conditions.
The αPT coefficient varies widely according to surface
type and condition (vegetation type and soil moisture condition) and micrometeorological
conditions such as strength of advection (Eaton et al., 2001; Fisher
et al., 2005). Pereira (2004) reported that the αPT
parameter can be set equal to the inverse of the McNaughton-Jarvis decoupling
factor. A wide range of values for αPT has been determined
for different types of terrains. Some of these suggested values are summarized
in Table 1.
||Suggested estimates of αPT for different
As it is shown in Table 1, αPT values approach
to unity in wetland areas. Eaton et al. (2001) represented αPT
values for five terrain types. They showed that αPT values of
less than unity to 0.77 are suitable for coniferous forests and values up to
2.32 for deep lakes. Wang et al. (2004) found a range of 1.1 to 1.4 for
αPT from HAPEX-SAHEL data sets. Flint and Childs (1991) reported
values ranging from 0.7 to 1.6. For a study in the peatlands of Hudson Bay,
αPT was found to be 0.63.
Investigations on αPT showed that it increases for dry
surfaces. Chuanyan et al. (2004) reported from previous investigators
a value as large as 1.57 under strongly advective conditions. Jensen et
al. (1990) ascertained a value of 1.74 for arid and semiarid climates.
Chuanyan et al. (2004) used this value to model potential evapotranspiration
in the semiarid regions of China, the Zuli River Basin, successfully,
especially in areas with more homogeneous land cover. Xiaoying and Erda
(2005) showed that PT-model underestimates crop evapotranspiration in
annual and monthly scales especially when wind speed is high. In the modified
approach, αPT is a function of environmental variables.
And a unique, physics-based functional form for the modified it has not
been defined and must be determined empirically (Sumner and Jacobs, 2005).
It seems that in arid regions αPT must be very different
from common values because of large amount of energy advection. The Sistan
plain in the southeast of Iran is one of those regions that experiences
high amount of advection fluxes annually especially during summer time
(May to October). In this study, calibrating PT model to estimate open
water evaporation from the Chahnimeh Reservoir, the only regulated fresh
water reservoir of Sistan area, using a water budget method is introduced.
MATERIALS AND METHODS
The Sistan Plain is one of those regions that experiences high amounts
of advection fluxes when a regional continuous wind flow occurs annually
especially during summer time (May to October). This seasonal wind is
called the 120-day wind. The monthly average of maximum wind speed at
10 m height during this period is about 8 m sec-1 and in rest
of the year is less than 2 m sec-1 with dominant direction
of north to south and northwest to southeast. Sistan is an area subject
to hydro-meteorological extremes, such as large floods and severe persistence
droughts. There are not any considerable groundwater resources in this
part of Iran and the annual precipitation is less than 60 mm, whereas
records of class A evaporation pan shows annual evaporation of more than
4500 mm. The only water resource in this region is the Hirmand River,
a trans-boundary river between Iran and Afghanistan (Fig.
1). The Hirmand is the tenth-largest river in Asia and drains much
of Afghanistan. The main branch of the Hirmand forms the international
boundary between Iran and Afghanistan. And, the Hirmand water right is
the major geopolitical crises between the two countries. The Hirmand River
flows to the interconnected wetland system of Hamuns with a total area
of 4000 km2 for full condition. Near the Iran and Afghanistan
border line a reservoir has been constructed to supply water for domestic
and agricultural activities within the Sistan Plain.
The Chahnimeh Reservoir comprises of four large depressions located north
of the delta of Hirmand River in Sistan region. The geographical boundary
of the region is from 30° 45` to 30° 50` northern latitude and
61° 38` to 61° 45` Eastern longitude. The average altitude is
500 m above mean sea level.
||Sistan plain in southeast of Iran
An earthen dam lined with concrete blanket
has been constructed to close the depression in an appropriate elevation
(Fig. 2). The outlet structure of irrigation canal is
located on the dam.
This reservoir forms a continuous water body in the wet years and separates
water bodies in dry years. The Chahnimeh Reservoir with a maximum capacity
of approximately 630 mm3 (MCM) and an area of over 47 km2,
provides fresh water to Zabol (center of Sistan region), Zahedan (center
of Sistan and Baluchestan Province) and other adjacent inhibited areas
of Sistan; in other words, the Chahnimeh Reservoir supplies domestic water
for more than 1 million people residing in the region, as well as irrigation
water to roughly 80,000 ha of farmlands in the region. Only half of the
reservoir`s capacity is considered live and the rest is the dead volume.
Chahnimeh hydrology has become the subject of many researches by planners
and engineers to estimate open water evaporation and save water for the
expanding population as well as the whole ecosystem. Numerous studies
and projects were proposed to estimate evaporation from Hamun Wetlands
and Chahnimeh Surfaces.
Most of the past studies in the region relied on the computation of evaporation
using meteorological ground station data but some of them are on the base
of remote sensing and application of satellite imagery.
||Chahnimeh Reservoir and measuring sites
Daneshkar Arasteh et al. (2005) according to a statistical point
of view showed that evaporation from Chahnimeh Reservoir belongs to two
hydrological conditions, one for those years that Hamun Wetlands are full
of water (wet condition or HG2) and the others for which Hamuns are approximately
dried (dry condition or HG1). In Daneshkar Arasteh et al. (2005)
study, Hamuns open water area with more than 75% of the area was taken
as wet condition and in the rest of time it was taken as dry. They showed
that this statistical separation is fully adapted with climatic conditions
of the area. During wet conditions agricultural activities and irrigated
area is maximized and wetlands and irrigated area have the same effects
on Chahnimeh Reservior evaporation. Figure 3 shows average
monthly open water evaporation from Chahnimeh Reservoir and its deviation
using water budget of the reservoir.
Daneshkar Arasteh (2004) developed a remotely sensed model to estimate
Hamun Wetlands surface evaporation named HRSE (Hamun Remotely Sensed Evaporation
model). Daneshkar Arasteh (2005) used HRSE to partition energy balance
components in Sistan area. He showed advection has a value of nearly equal
to net radiation in this part of the world, an amount that has almost
never been seen anywhere. It seems that Sistan area and Hamun Wetlands
are very unique with many catastrophes. Daneshkar Arasteh and Tajrishy
(2006) used HRSE to estimate open water evaporation and vegetation cover
evapotranspiration from Hamun Wetlands.
A standard meteorological station is located on the northern bank of
the reservoir next to the outlet of the Chahnimeh Dam. Daily records of
meteorological data including min, max, wet and dry temperature, pan evaporation,
wind speed and direction could be found for more than 12 years (since
1994), in this station.
||Chahnimeh monthly evaporation (Daneshkar Arasteh et
Data from Zahak first order meteorological station,
located 5 km North of the reservoir is also used for control and completion
of Chahnimeh station data. Zahak station began to record from 1992. In
Zahak meteorological station sun shine hour and shortwave radiation flux
density are collected in addition to normal data of Chahnimeh station.
All of parameters measured hourly.
Daily inflow and outflow are measured at two ordinary hydrometric stations.
Inflows are measured in Jarikeh, a station equipped with a water level
recorder, a telepheric bridge, a 4 m height stage and a standard AOTT
current meter. Water surface levels in the Chahnimeh feeder canal are
measured continuously and inflow volume integrated and reported daily
Outflows are measured at the outlet of the dam with a 4 m height stage
at upstream of the outlet weir, twice a day. Lake surface level is also
measured twice a day by a set of eight numbers of 1 m height stages. Daily
pumping volumes of the Zabol and Zahedan Pumping Plants are determined
on the basis of their rating curves.
In this research, an eleven-year duration from May 1994 to September
2004 is considered, in which dry, normal and wet spells are included.
Water budget of Daneshkar Arasteh et al. (2005) study was considered
and free water evaporation was determined for the whole water year. This
water budget on the base of a Volume balance Equation (VB) for five day
periods was used. Meteorological data of Chahnimeh and Zahak stations
were also used to model free water evaporation by PT method. To calibrate
αPT, ratio of evaporation rates from equilibrium method
and VB was determined.
RESULTS AND DISCUSSION
As mentioned earlier, it seemed that common models require calibration
before application in the study area. Figure 4 and 5
show the free water evaporation from Chahnimeh using the VB and PT methods
before calibrating for two wet (HG2) and dry (HG1) hydrological conditions
for May to October period. As it is shown from Fig. 4,
PT model estimates free water evaporation less than true evaporation for
dry years during May to October when the 120 day wind blows a warm dry
wind blowing over the reservoir. In other words, PT method underestimates
open water evaporation from Chahnimeh surface within the periods that
Hamun Wetlands are approximately dried. Therefore, it requires calibration
for these periods.
But, Fig. 5 shows relatively good estimations. Slope
of trend lines for the relation between VB and PT results are 0.51±0.02
and 1.05±0.03, respectively for hydrological conditions of HG1
and HG2. These values show that during dry condition, PT model under predicts
the evaporation rate about 50%.
Therefore, αPT values (ratio of evaporation rates from
equilibrium method and VB) varying between 2.47±0.09 and 1.20±0.03
for those conditions, respectively. It is shown that when Hamun Wetlands
are full of water, Chahnimeh Reservoir acts similar to many other lakes
and reservoirs all around the world and PT method leads to reasonable
estimates for open water evaporation. But, during the periods in which
Hamun Wetlands are dried this method underestimates the evaporation rate
and evaporative behavior of the Chahnimeh Reservoir changes.
Comparison between free water evaporation from Chahnimeh
Reservoir measured by volume balance (VB) and estimated by Priestley-Taylor
(PT) methods for dry hydrological condition (HG1) from May to October
Comparison between free water evaporation from Chahnimeh
Reservoir measured by volume balance (VB) and estimated by Priestley-Taylor
(PT) methods for wet hydrological condition (HG2) from May to October
It is clear
that under dry conditions the coefficient is increased about two times
of normal conditions (2.47±0.09). The value of αPT for HG2 conditions (1.20±0.03) is similar to ones for other lakes
and reservoirs around the world such as those summarized by Eaton et
Using free water evaporation rate from Chahnimeh surface determined by
VB method as the given variable to PT model and using meteorological data,
coefficient of αPT was calibrated for monthly time scale.
Figure 6 shows temporal variation of αPT
within May to October for two statistically homogeneous groups or hydrologic
||Monthly variation of αPT for Chahnimeh
Figure 6 shows, αPT varies from 0.73
to 2.10 during May to October. For the HG1 group, it is larger than the
HG2 group (1.56 to 2.10 for HG1 against 0.73 to 1.13 for HG2). It is observed
that from May to October, in which the 120 day warm wind blows and regional
energy advection occurs, the Chahnimeh Reservoir experiences conspicuous
rise in αPT.
From Fig. 6, one could find that the evaporation from
the reservoir decreases to a little more than 70% of equilibrium evaporation
rate, for May for the HG2 group and approaches to unity for wet conditions.
A similar behavior is seen for the HG1 group but it is less intense. Figure
6 also shows that the decrease in αPT in the HG2 group
is more than that of the HG1 group in similar months. The reason should
be sought in the air content of sensible heat. The HG1 group is related
to dry years, during which Hamun Wetlands do not usually have any water.
Therefore, as the 120 day wind blows, it carries an enormous amount of
energy as sensible heat from upstream middle latitudes and desert lands
of Iran`s central plateau to Sistan region and highly influences the evaporation
from the reservoir surface and augments it seriously. Whereas during the
HG2 years, a part of this energy is consumed for evaporating from the
surface of the Hamun Wetlands as the wind first flows over the wetlands
water bodies with an area of 4000 km2 during the wet years.
In other word, advected sensible heat transfers to latent heat over Hamun
Wetlands. On the other hand, the elimination of sensible heat over the
Hamun Wetlands and evaporation from their surfaces increase the regional
relative humidity and this causes a decrease in vapor pressure deficit.
Both factors decrease the evaporation from the surface of the Chahnimeh
As Sene et al. (1991) confirmed, αPT = 1.26 is
equivalent to suggesting that, on average, for a water surface, the aerodynamic
term of a combination model such as Penman equation contributes 21-22%
of the total evaporation whereas in Chahnimeh Reservoir, obtained values
for αPT, showed a higher aerodynamic term. This larger
term is related to the 120 day wind and transferring large amount of energy
by air flow. Unfortunately, there were no proper equipments to measure
sensible heat displacement in the study area.
Evaporation is the most important unknown factor in the water budget
of the Chahnimeh Reservoir, Sistan region, Iran. This study showed that
during the year from May to October (when the 120 day warm wind blows
through the Sistan), αPT increases due to increase in
advected sensible heat. This increase in the αPT is less
in the wet years in which nearby the Hamun Wetlands are approximately
full of water against dry conditions which their area decreases dramatically.
It was shown that common well known models such as Priestley-Taylor requires
calibration in the area. Calibration showed the Priestley-Taylor coefficient
is time dependent and affected by season and existence of water in Hamun
Further study of the other methods such as mass transfer and energy budget
are highly recommended. And also, it is recommended to carry an investigation
on energy advection distribution and atmospheric circulation in the Sistan
area and its influence on water surface evaporation. But, first of all,
it is needed to equip the hydrometric and meteorological stations with
required measuring devices as well as a floating energy balance station
on the reservoir.