INCREASED LOADING OF ATMOSPHERIC WATER VAPOR:
HYDRO-METEOROLOGICAL CONSEQUENCES
R.D. DESHPANDE
Physical Research Laboratory
Navrangpura, Ahmedabad 380 009. India.
S.K. GUPTA
Physical Research Laboratory
Navrangpura, Ahmedabad 380 009. India.
The magnitude of evaporation from land areas is set to increase significantly in some
regions due to large scale engineered systems involving huge surface reservoirs and inter
basin transfers of water and its use in irrigation. Given the limited atmospheric residence
time (8-9 days), increased vapor loading can have corollary increase in precipitation.
However, due to regional aspects of atmospheric circulations, both increase and decrease of
precipitation are possible. In this paper, following a discussion on the available data
showing increased atmospheric water vapor loading and consequent changes in the pace of
hydrological cycle, an outline of a multidisciplinary approach to address this issue in respect
of India is presented.
INTRODUCTION
Water vapor is the major greenhouse gas (GHS) in the atmosphere and responsible for
>60% of the natural GHG warming (20.6 0C of the total of 33 0C). While atmospheric water
vapor significantly contributes to natural greenhouse effect, the climatic impact of warming
induced increase in vapor loading is believed to be only a few percent [1]. Since water
vapor can cause both radiative heating and cooling, additional loading may not have a net
feedback effect to modify the earth’s temperature [2]. But, it may accelerate the
hydrological cycle both globally and regionally and modify the hydro-meteorology
significantly. Therefore, any increase in atmospheric vapor content and observed changes in
precipitation and stream flow need to be viewed more seriously by hydro-meteorologists
than by climatologists. Due to increase in tropical sea surface temperature during the last
30-40 years, an annual increase of evaporation (and hence vapor loading) of ~1.2x10 18 g
H2O/yr is calculated [1]. This increase perhaps is insignificant, being only ~0.23% of the
average annual global evaporation (525x1018 g H2O/ yr). Similar to oceanic areas,
evaporation from land areas might increase regionally in response to warming and
irrigation. The magnitude of evaporation from land areas is set to increase significantly in
some regions due to large scale engineered systems involving huge surface reservoirs and
inter basin transfers of water and its use in irrigation. Given the limited atmospheric
residence time (8-9 days), increased vapor loading can have corollary increase in
precipitation. However, due to regional aspects of atmospheric circulations, both increase
(Northern Hemisphere mid-and high latitudes) and decrease (tropics and subtropics in both
hemispheres) of precipitation are possible [2]. For example, some indications of increased
precipitation possibly due to enhanced evapo-transpiration (ET) in the command areas of
Indira Gandhi Canal Project in Rajasthan, India have been reported [3,4]. Similarly, the
observed [2] increase in precipitation, increased frequency of extreme rainfall and stream
flow in certain parts of world might be regional effects of increased vapor loading.
Thus, both GHG and irrigation induced increase in evaporation can have far reaching
consequences on hydrology, meteorology and agro-ecology of a region and influence its
socio-economy. However, due to variability of vapor content in time, location and altitude,
and its linkages to the atmospheric dynamics, the regional consequences of additional vapor
loading are too complex to describe.
In this paper, following a discussion on the available data showing increased
atmospheric water vapor loading and consequent changes in the pace of hydrological cycle,
an outline of a multidisciplinary approach to address this issue in respect of India is
presented.
PARADOX OF ATMOSPHERIC WATER VAPOR
Based on various scenarios of increasing atmospheric concentrations of GHG and aerosols,
the mean annual global surface temperature is projected to increase by 1° to 3.5°C by year
2100 [2]. The rate of evaporation from oceans and hence atmospheric water vapor loading
are sure to increase due to increased global surface air temperature and because the water
holding capacity of air is a temperature dependent property. While there are ample
evidences of increased atmospheric vapor content in the different parts of the world, the
terrestrial pan evaporation data from the digital pan evaporation network (746 stations) of
the US and evaporation pan reporting stations network (190 stations) of the former USSR
showed a decreasing trend [5]. Similar decreasing trends in pan evaporation were also
reported for India [6] and Venezuela [7]. These downward trends in pan evaporation were
interpreted to imply that for large regions of the globe, the terrestrial evaporation
component of the hydrological cycle had been decreasing due to increasing cloudiness.
These inferences were paradoxical to well-sustained increase in the global precipitation and
cloudiness [8]. Subsequently, Brutsaert and Parlange [9] demonstrated that in non-humid
environments, measured pan evaporation is not a good measure of potential evaporation.
Moreover, in many situations, decreasing pan evaporation actually provides a strong
indication of increasing terrestrial evaporation. The relationship between actual evaporation
(E), potential evaporation (Eo) and pan evaporation (Epa) is dictated by the availability of
land-surface moisture and the heat energy. In a situation where land-surface moisture is in
ample supply and available energy is the limiting factor, E, Eo, and Epa have the same value
(i.e. E = Eo = Epa). When the availability of land-surface moisture is limited, the E < Eo and
the available excess heat energy manifests as the increased sensible heat flux (H = Eo - E).
Under such conditions Epa exceeds Eo according to relationship aEpa = Eo + bH; where a (≈
1) and b (≥1) are pan coefficients. Eliminating H in the above, yields relationship (E =
[(1+b)Eo - aEpa]/b) between E, Eo and Epa. This relationship shows how observed decrease
in pan evaporation means increase in actual evaporation. This is plausible because the rate
of evaporation at particular temperature depends on the difference between the saturation
vapor pressure and actual vapor pressure of the air. In otherwise arid region, the availability
of additional surface water increases the vapor pressure, thereby reducing the difference
between it and the saturation vapor pressure. Under such a condition, the rate of evaporation
(observed from Epa) decreases although the overall evaporation increases. This is consistent
with data [8,10,11] indicating an intensification of hydrological cycle in large regions
where, increasing precipitation leads to increasing surface run-off and soil wetness, which in
turn generates more evaporation.
The observed increase in atmospheric water vapor loading in response to global
warming poses two fundamental questions: whether the increasing atmospheric vapor
loading is sufficient to (i) force the additional climate shift and (ii) modify the regional
hydrological cycle.
There has been a general consensus that the increased atmospheric vapor loading due
to GHG emissions will not be sufficient to cause the additional climate shift [2]. This is
explained by the fact that the possible warming due to greenhouse effect of increased vapor
is counter balanced by processes such as, radiative cooling due to increased cloudiness and
drying up of the air mass in the descending branches of the Hadley cell [12]. As a result of
such bi-directional temperature feedback of vapor, increased atmospheric vapor loading is
believed to cause no additional temperature shift.
Though no major climatic shifts are anticipated, the increased atmospheric vapor
loading is believed to modify the hydrological cycle regionally. A quantitative overview of
the global hydrological cycle can offer clues to; (i) the possible changes that can be
anticipated in the hydrological cycle; (ii) sites where such changes can be best observed and
(iii) possible tools to observe such hydrological changes.
GLOBAL WATER BUDGET AT A GLANCE
The global hydrological cycle in terms of annual turnover, storage in three main reservoirs
namely atmosphere, ocean and Land, annual fluxes between them and residence time is
shown in Figure 1. It is seen that the total water vapor turn-over in the global hydrological
cycle is ~577x103 km3 of which 502.8x103 km3 originates from ocean and 74.2x103 km3
originates from land (~0.5m/yr). About 91% of vapor originating from ocean surface
precipitates over ocean and ~9% is carried over land. This oceanic vapor carried over land
contributes ~37% of precipitation over land with ~63% contribution from land derived
vapour [13]. Since the residence time of water vapor in atmosphere is only ~9 days, the
increased evaporation both in oceanic and land areas and resultant increase in atmospheric
water vapor loading can increase the precipitation over land regionally.
Figure 1. A quantitative overview of the global hydrological cycle showing storage in
different reservoirs, annual fluxes between them and residence time in each reservoir.
All the volume and flux values are in 103 km3 units. Storage volumes are in bracket.
(Data compiled from [13, 14])
IMPACTS OF INCREASED VAPOUR LOADING: REGIONAL STUDIES
With increase in irrigation required to meet increased food demand for the growing
population, very significantly in China and India along with other countries, the atmospheric
loading of vapours over land is set to increase significantly. For example in India, over the
next fifty years, human water use induced increased vapor loading of atmosphere may
increase by about 0.2m/yr. Even so, considering the volumes involved its impact on the
GHG induced hydrological change on global scale is difficult to predict. But surely the
impact will be significant at regional scale. To get some idea of the kind of regional impact,
one would like to draw inference first from a study wherein human water use is not taken in
consideration and only the GHG climate scenarios have been considered. Gosain et al [15]
used SWAT (Soil and Water Assessment Tool) water balance model for all river basins of
India excluding Brahamputra, Indus and the snow bound area of Ganga. In this study, daily
weather generated by HadRM2 was used to simulate control climate scenario (1981-2000),
which was compared with GHG climate scenario (2041-2060) without changing the land
use. The outputs of these two scenarios were analyzed with respect to possible runoff, soil
moisture and actual ET. Within these limitations, the study (Figure 2 reproduced from [15])
shows that simulated impacts were different in different catchments. The increase in rainfall
was seen in Cauvery, Brahmni, Godavari, Mahanadi, and Ganga. Whereas the rainfall was
seen to decrease in other basins, namely, Krishna, Luni, Mahi, Narmada, Pennar, Tapi and
Sabarmati. But the increase or decrease of rainfall did not necessarily lead to increase or
decrease of surface runoff or the increase or decrease of evapo- transpiration. But the
important point to notice is that the estimated increase/decrease in ET is very small
(generally <0.05m/yr). Associated with these hydrological components were changes in
terms of severity of droughts and intensity of floods in various parts of the country.
Generally speaking the GHG scenario indicated deteriorating conditions both for drought
and floods (i.e. increase incidence of extreme events) with general reduction in the quantity
of available runoff in the GHG scenario.
Figure 2. Trend in water balance for Control and GHG climate scenarios (reproduced
from Gosain et al [15].
It is interesting to compare these results with a study on the stream flow trends in US.
In one study [16] secular trends in streamflow were evaluated for 395 climate sensitive
stream gauging stations in the coterminous US using non parametric Mann-Kendall test.
The trends were calculated for selected quantiles of discharge, from 0 th to 100th percentile to
evaluate differences between low-, medium- and high-flow regimes during the 20th century.
Two general patterns emerged: trends were most prevalent in annual minimum to medium
flow categories and least prevalent in the annual maximum category. At all but the highest
quantiles, streamflow showed increase across broad sections of the US. Decrease appeared
only in parts of the Pacific Northwest and Southeast. Hydrologically these results indicated
that coterminous US was getting wetter, but less extreme.
In another study from US [10] the proportion of total precipitation contributed by
extreme one-day event was seen to have increased significantly during the twentieth
century. The reported increase in precipitations was modest, although concentrated in
higher quantiles. This study lent support to the climate model simulations indicating
intensification of hydrologic cycle in response to increased GHG emissions, particularly to
mean more extreme hydrologic events such as flood and droughts.
An example of a study where increased human use of water may have played a role
in modifying the hydrologic cycle comes from a study [17] of the Mississipi River basin
(area 3x106km2) – comparable in area with India (3.29x106km2). Data revealed an upward
trend in evaporation during recent decades driven primarily by increase in precipitation and
secondarily by human water use. A cloud-related decrease in surface net radiation appears
to have accompanied the precipitation trend. Resultant evaporative and radiative cooling of
the land and lower atmosphere quantitatively explained downward trends in pan
evaporation. Contrary to the anticipated GHG warming, the observed cooling tendencies,
have been explained as forced by the North Atlantic Oscillation and an eventual return to
normal precipitation could reveal so far unrealized GHG warming in the basin. If, however,
the increased precipitation was caused by some unidentified forcing (e.g. increasing human
use of water), then continued intensification of water cycling and suppression of warming in
the basin could continue. The uncertainty is due the fact that increases in atmospheric
moisture supply due to consumptive use within and outside the basin was far too small to
explain observed precipitation increase.
We thus see that increased loading of atmospheric water vapor either as a result of
GHG warming of the atmosphere or due to human consumptive use will have significant
hydro-meteorological consequences both regionally and globally. Regionally, the impacts of
human consumptive use may over-ride the global trend because water vapor unlike other
GHG has short residence time in the atmosphere and its movement is largely controlled by
meteorological factors.
UNDERSTANDING THE HYDROLOGY OF INDIA
The population of India is expected to stabilize around 1,640 million by the year 2050. As a
result, gross per capita water availability will decline from ~1,820 m3/yr in 2001 to as low
as ~1,140 m3/yr in 2050. Total water requirement of the country for various activities
around the year 2050 have been assessed to 1,450 km3/yr [18]. This is significantly more
than the current estimate of utilisable water resource potential (1,122 km3/yr) through
conventional development strategies. Therefore, when compared with the availability of
~500 km3/yr at present the water availability around 2050 needs to be almost trebled.
Whichever way this water demand is met, the loading of atmosphere by additional water
vapour is imminent to the extent of ~0.2m/yr [19]. For a sustainable development with high
degree of resilience, particularly when the working at a level wherein large scale inter-basin
transfer of water for consumptive uses becomes a must, it is important to understand and
manage the hydrological cycle in great detail. Towards this statistical analysis of time series
of rainfall, pan evaporation, temperature and streamflow provide important clues. Further, if
these investigations are combined with spatial and temporal monitoring of stable isotopes in
different components of the hydrological cycle, we can trace sources of atmospheric water
vapor, rain, actual evaporation from surface water bodies and transpiration from plants,
identify streamflow components in different seasons to precipitation and/or groundwater
discharge from different parts of the catchments.
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