Sensitivity of the global water cycle to the water-holding capacity of soils

Transcription

1 Exchange Processes at the Land Surface for a Range of Space and Time Scales (Proceedings of the Yokohama Symposium, July 1993). IAHS Publ. no. 212, Sensitivity of the global water cycle to the water-holding capacity of soils P. C. D. MILLY US Geological Survey, Geophysical Fluid Dynamics Laboratory/NOAA, PO Box 308, Princeton, New Jersey 08542, USA Abstract A numerical experiment using an atmospheric general circulation model was employed to estimate the sensitivity of the global water cycle to the water-holding capacity of soils. An increase of the globally constant capacity from 4 cm to 60 cm yielded an increase (24 cm) of evaporation from land, a decrease (-11 cm) of runoff, and an increase (13 cm) of precipitation onto land. Decreased runoff was balanced by decreased water-vapor flux convergence over the continents. In the tropics, the induced weakening of the monsoonal circulations was the major factor in this net export of water vapor to the oceans; in the middle latitudes, moistening of continental air masses, with resultant reduction in onshore vapor transport by transient eddies, was the dominant mechanism. INTRODUCTION The critical hydrologie characteristic of the continents, from a climatic standpoint, is their ability to absorb water from precipitation, to store it, and later to release it to the atmosphere. In so doing, the continents play an active role in the global cycling of water and energy. The simplest measure of this hydrologie action of the continents is the water-holding capacity of the soil. The purpose of this study was to analyze the equilibrium hydrologie and thermal sensitivity of a model atmosphere to the water-holding capacity of continental soils. Topics of special emphasis were the direct sensitivity of the surface water balance to the soil water-holding capacity, the role of atmospheric feedbacks to the surface, and the aspects of atmospheric response relevant to horizontal water vapor transport. In particular, the feedback of circulation changes on water vapor transport was quantitatively examined. A brief summary of selected results is presented here. A recent series of studies of potential climatic effects of tropical deforestation (Dickinson & Henderson-Sellers, 1988; Lean & Warrilow, 1989; Shukla et al., 1990) a_re related to the present work. Those studies examined the effects of prescribed changes in the soil and vegetation characteristics that affect surface albedo, surface roughness length, and water-holding capacity. However, they did not provide an understanding of the separate role played by changes of soil water-holding capacity, a_nd they had a limited geographic focus. The water-holding capacity of soils is known to depend on soil hydraulic characteristics and on plant root depth and density. The resulting large-scale geographical variability of water-holding capacity (Patterson, 1990) is recognized, either explicitly (e.g. Hansen et al., 1983) or implicitly (as in the models used for the

2 496 P. C. D. Milly deforestation experiments cited above), in many general circulation models used for climate simulations. For the study described here, whose purpose was to determine sensitivities and to elucidate the physical mechanisms that control them, the advantages of a globally constant water-holding capacity - simplicity and clarity of analysis - seemed to outweigh the disadvantage marginal degradation of the model hydrology and climatology. PLAN OF NUMERICAL EXPERIMENTS The numerical experiments were conducted using a general circulation model of the atmosphere, coupled to simple energy- and water-balance parameterizations of the continental surfaces, with prescribed ocean-surface temperatures. The dynamical component of the general circulation model, described by Gordon & Stern (1982), approximately solves transport equations for vorticity, divergence, temperature, and water vapor mixing ratio. The numerical approximations are generated by forming finite differences in time and in the vertical space dimension, and by representing horizontal variability by means of a finite sum of spherical harmonics. The time step is 30 minutes; there are nine vertical levels; and rhomboidal-15 truncation is used for the spherical harmonics. Land-surface processes and their interactions with the atmosphere are computed in the space domain, using a grid having a longitude spacing of 7.5 and a latitude spacing that averages 4.5. The cloud distribution, employed in radiative transfer calculations, was prescribed as height- and latitude-dependent zonal means based on observations. Seasonal, but not diurnal, variability of solar forcing at the top of the atmosphere was included. Continental surface processes were described using Manabe's (1969) implementation of Budyko's (1956) land-surface water- and energy-balance model. The storage of soil water is calculated at each time step by an accounting of snowmelt, liquid precipitation, evaporation, and runoff. Runoff occurs when (and only when) saturation of the storage capacity prevents further storage of water. Evaporation is given as the product of a potential rate and a simple function of storage; the potential rate is calculated under the assumption that the air adjacent to the surface is saturated at the surface temperature (Milly, 1992). Surface temperature of the continents is determined from a balance among radiation and sensible and latent heat fluxes. Two climate simulations (herein designated W4 and W60) were performed, differing only in the assignment of globally-constant soil water-holding capacities. Values of 4 cm (W4) and 60 cm (W60) were used. Both simulations were run from an initial condition of an isothermal atmosphere and zero soil water storage. Equilibrium was reached after two years of simulation in the 4-cm case and after 8 years in the 60-cm case. Analyses were based on 10-year simulations following these initial spin-up periods. RESPONSE OF CONTINENTAL SURFACES The direct effect of an increase in storage capacity at the land surface is to increase the ability of the soil to save water from periods of water excess and to return it to the atmosphere at times of water shortage. In the annual mean, this translates to an

3 Sensitivity of the global water cycle to the water-holding capacity of soils 497 Fig. 1 Global distribution of difference (W60-W4) in annual evaporation (cm). Fig. 2 Global distribution of difference (W60-W4) in annual runoff (cm). increase in evaporation (Fig. 1) and a decrease in runoff (Fig. 2). Resulting atmospheric feedbacks drive additional, indirect effects on surface water balance. For the current experiment (Fig. 3), annual evaporation from land increased by 24 cm (from 54 cm to 78 cm) on average, and annual runoff dropped by 11 cm (from 39 cm to 28 cm). The difference between these amounts is due to a 13 cm increase (from 95 cm to 108 cm) in precipitation over land. A separation of direct and indirect (atmospheric feedback) effects was made on the basis of an analytic model of the annual surface water balance. Direct hydrologie response In the tropics, the change in storage capacity was felt mostly over short time scales. During the dry season on the fringes of the intertropical convergence zone, the storage capacity was critical for saving water from storm events until periods of fair weather. Seasonal storage was not a major factor, because seasonality of precipitation and potential evaporation were not great. In the northern middle latitudes, seasonal storage was a major factor determining the sensitivity of the annual water balance to the water-holding capacity. Unlike the

4 498 r I _ ; P E r I 2.4 ) P. c. D. Mi7fy 14.7 } Land J (W4 + W60) / 2 r I c v Ï Atmosphere J E P Ocean y Atmosphere Atmosphere P 12.7 E 24.2 E -2.5 P 2.3 Land Ocean W60 - W4 Fig. 3 Major annual fluxes (cm) in the global water cycle. Top: (W4 + W60)/2. Bottom: (W60 - W4). 4-cm case, the 60-cm case permitted interseasonal transfer of excess winter precipitation to satisfy the high summer demand for evaporation. The ability of the larger capacity to conserve heavy summer rainfalls was also a major factor in the water balance. Atmospheric feedbacks on hydrology The changes in surface water balance were also affected by atmospheric feedbacks. The increased evaporation led to increases in precipitation (by mass balance) and decreases in potential evaporation (by moistening and cooling the lower atmosphere). Both of these atmospheric responses to the change in soil water-holding capacity tended to increase runoff. In the tropics, these negative feedbacks were about one-third of the direct effect on runoff. Farther north, their sum approached the magnitude of this direct effect, leading to large areas over which annual runoff actually increased as a result of an increase in storage capacity. In contrast, the net atmospheric feedbacks on changes in evaporation were relatively small, because of cancellation of the influences of the increase of precipitation and the decrease of potential evaporation. It was noted that the 60-cm case represented a limiting case, in that a 60-cm storage capacity was sufficient to provide virtually all temporal balancing of water and energy supplies affecting evaporation from the continents. In that simulation, runoff was produced only at those locations where the annual precipitation exceeded the annual potential evaporation, and the amount of runoff was equal to their difference.

5 Sensitivity of the global water cycle to the water-holding capacity of soils 499 RESPONSE OF ATMOSPHERE Precipitation The increased evaporation from the continents caused equal increases in global precipitation. In each season, the zonal mean changes in evaporation and precipitation were approximately equal, with maxima in the northern middle latitudes and in the tropics (Fig. 4). In the middle latitudes, large-scale changes of evaporation and precipitation were broadly similar, though continental precipitation changes were somewhat smaller due to transport of some water vapor offshore. In North America, Europe, and northern Asia, the summer precipitation increased by 10 to 20 cm. Relatively small increases in precipitation were computed over the North Pacific and North Atlantic Oceans. In the tropics, differences between distributions of changes in evaporation and changes in precipitation were notable. Although the evaporation increases were concentrated under the downward branches of the Hadley circulation, the precipitation increased most under the upward branches. Along the equator, the annual increases in continental precipitation fell far short of the evaporation increases and were even negative over a large part of South America. Correspondingly, large increases in precipitation were computed over the equatorial Atlantic and Indian Oceans. Much of the difference between tropical changes in precipitation and evaporation can be understood in terms of prevailing atmospheric circulation in the tropics. Water vapor added to the atmosphere outside the intertropical convergence zone (ITCZ) was transported by surface winds toward the ITCZ, where rising motion caused it to condense as precipitation. Atmospheric heating and tropical circulation The increase of evaporation under the downward branches of the Hadley circulation led to a net reduction of heating of the atmospheric column there, and the increase of precipitation from the upward branch led to a relative heating. Both of these changes in heating drove an intensification of the Hadley circulation. Fig. 4 Global distribution of difference (W60-W4) in annual precipitation (cm).

6 500 P. C. D. Milly In the annual mean, heating increased over the oceans because of added precipitation and decreased over the continents because of decreased convergence of water vapor transport. This led to a reduction in the land-sea heating contrast that drives the monsoon circulations. The weakening of these circulations induced a differential offshore transport of water vapor. Water vapor transport In the tropics, the intensification of the Hadley circulation increased the meridional transport of water vapor toward the ITCZ, strengthening the increases of precipitation at the ITCZ and reducing the increase of precipitation under the downward Hadley branches. The weakening of the monsoonal circulations decreased the transport of water vapor from the ocean regions to the continental regions. This explains the relatively weak increases in precipitation over the tropical continents and the strong increases over the oceans. Over the continents in middle latitudes, where mean circulation is not so sensitive to heating as in the tropics, there were, nevertheless, significant reductions in the convergence of atmospheric water vapor fluxes. A detailed analysis of the water vapor fluxes revealed that these changes were associated with the transient eddy fluxes of water vapor, which in turn were explained by the reduction in the land-sea humidity gradient. This reduction can be understood by noting that the increased evaporation from the continents led to more moistening of the atmosphere over land than over ocean; changes in column-integrated water vapor content were well correlated with changes in precipitation. DISCUSSION One practical application of the work described here is in the estimation of the sensitivity of climate models to errors in surface hydrologie parameterization. The soil water-holding capacity employed here may be viewed as a surrogate for the general tendency of any surface hydrologie parameterization to save excess water for future evaporation at the expense of current runoff. Thus, in a model that favors evaporation excessively, there will be a tendency toward insufficient runoff, excessive mid-latitude near-surface humidity, an overly intense Hadley circulation, weakened monsoons, and insufficient onshore transport of water vapor at all latitudes. A parameterization that favors runoff excessively will have opposite effects. REFERENCES Budyko, M. I. (1956) Heat Balance of the Earth's Surface (in Russian). Gidrometeoizdat, 255 pp. Dickinson, R. E. & A. Henderson-Sellers (1988) Modelling tropical deforestation: A study of GCM land-surface parameterizations. Quart. J. Roy. Met. Soc. 114, Gordon, C. T. & W. F. Stern (1982) A description of the GFDL global spectral model. Mon. Weath. Rev. 110, Hansen, J., G. Russell, R. Rind, P. Stone, A. Lacis, S. Lebedeff, R. Reudy & L. Travis (1983) Efficient threedimensional global models for climate studies. Mon. Weath. Rev. Ill,

7 Sensitivity of the global water cycle to the water-holding capacity of soils 501 Lean, J. &D. A. Warrilow (1989) Simulation of the regional climatic impact of Amazon deforestation. Nature 342, Manabe, S. (1969) Climate and the ocean circulation, 1. The atmospheric circulation and the hydrology of the earth's surface. Mon. Weath. Rev. 97, Milly, P. CD. (1992) Potential evaporation and soil moisture in general circulation models. /. Climate 5, Patterson, K. A. (1990) Global distributions of total and total-available soil water-holding capacities. M S Thesis, Univ. Delaware. Shukla, H., C. A. Nobre & P. J. Sellers (1990) Amazonia deforestation and climate change. Science 247,

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