Transcript Lect 5
“The highest form of human intelligence is the ability to observe without judging” Krishnamurti “The intuitive mind is a sacred gift and the rational mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift” Albert Einstein “The mind is everything, what you think you become” Buddha U6115: Climate & Water Tuesday, August 16 2005 • Water Properties • • • Heat capacity latent heat saturation vapor pressure • • Evaporation transpiration, mass/energy balance • Evaporation • Droughts • Physical and historical impacts ...Now the wind grew strong and hard, it worked at the rain crust in the corn fields. Little by little the sky was darkened by the mixing dust, and the wind felt over the earth, loosened the dust and carried it away. The Grapes of Wrath, John Steinbeck. Heat Capacity – Specific Heat Land vs Ocean breeze Heat Capacity Substance Heat Capacity (Calorie/g.C) Water 1.00 Sea water 0.94 Air 0.25 Granite 0.20 Change in amount of heat (Q): DQ DQ = M x Cp x DT But Mass = rw x Volume DQ = rw x V x Cp x DT Latent Heat - Change of phase Liquid Solid = 80 calories per gram Liquid Gas = 540 calories per gram DQ = rw x V x CL Evapotranspiration evapotranspiration summarizes all processes that return liquid water back to the atmosphere into water vapor - evaporation: direct transfer of water from open water bodies - transpiration: indirect transfer of water from root-stomatal system • Photosynthesis requires water as well as solar energy • of the water taken up by plants, ~95% is returned to the atmosphere through their stomata (only 5% is turned into biomass!) • potential evaporation (PE), i.e. the evaporation rate given an unrestricted water supply - different from actual evaporation • how can the actual evapotranspiration be measured? • water balance • energy balance • or combination of both Evapotranspiration Apart from precipitation, the most significant component of the hydrologic budget is evapotranspiration. Evapotranspiration varies regionally and seasonally; during a drought it varies according to weather and wind conditions Slightly more than 10% of atmospheric moisture (40,000 bg) is precipitated as rain, sleet, hail, or snow in the conterminous USA. The disposition of this precipitation is illustrated below. Evapotranspiration: ~ 67% (majority of loss through transpiration: 97%) Runoff: 29% Groundwater outflow: ~2% Consumption: ~2% Evapotranspiration Estimates of average statewide evapotranspiration for the conterminous United States range from about 40% of the average annual precipitation in the Northwest and Northeast to about 100% in the Southwest. During a drought, the significance of evapotranspiration is magnified, because evapotranspiration continues to deplete the limited remaining water supplies in water bodies and soils Evapotranspiration Estimation of ET 1) from the water balance this approach may suffer from the uncertainties in the numbers, example: dV/dt = p + rsi - rso - et = 0 et = p + rsi - rso p = 107±5105 m3/y (±5%) rsi = 109±1.5108 m3/y (±15%) rso = 9.95108±1.5108 m3/y (±15%) Here, if we neglect the groundwater inflows and outflows, we can use these values to solve for et. The results, accumulating the errors as we go, is: 1.5107±3108 m3/y Unrealistic to expect to be able to quantify accurately all terms in a water balance for a catchment to solve for et, especially over short periods where storage changes are both substantial and difficult to measure precisely (or predict). Diagnostic NOT predictive approach Evapotranspiration Estimation of ET 2) from the Energy balance First Law of Thermodynamics: conservation of energy (E) Thermodynamic principles hold that the net radiant energy arriving across the boundary of a surface land system (including a very thin top soil layer, vegetation, and immediate surrounding air), must be exactly balanced by other energy fluxes across the boundary and the net change in energy held within the volume. Total incoming E = Outgoing E + any increase in the body’s internal E (DQ) dQ/dt = Rn - G - H - El Rn = net (solar) radiation G = output (conduction) to the ground H = output (sensible heat) to atmosphere El = output of latent heat Evapotranspiration Estimation of ET 2) from the Energy balance All matter has internal energy (expressed in calories or joules) a) Sensible heat is the portion of internal energy that is proportional to temperature (heat sensed by contact). The specific heat capacity provides a measure of how a substance’s internal energy changes with temperature dEu = Cp m dT Cp = dEu/(dT m) Water has a specific heat of 1.0 cal/g.°C or 4.2x103 J/kg.°C b) Latent heat is the amount of internal energy that is released or absorbed during phase change (no change in temperature), at a constant temperature. lv = 2.5 - (2.18 10-3 DT) 106 J/kg At 20°C lv = 2.45x106 J/kg Evapotranspiration Estimation of ET 2) from the Energy balance The rate of evaporation can be described, in the context of the energy balance equation, as an energy flux dQ/dt = Rn - G - H - El or El = Rn - G - H - dQ/dt Since the heat flux is related to the rate of evapotranspiration (through latent heat of vaporization) et = El/(rwlv) We can then substitute this later equation into the previous one: et = (Rn - G - H - dQ/dt)/(rwlv) Evapotranspiration Estimation of ET 2) from the Energy balance Example: Daily evaporation from a forest on a sunny day (Rn = 200 W/m2) et = (Rn - G - H - dQ/dt)/(rwlv) If we can neglect H and G and assume that T (thermal energy content, Q) within the forest remains approximately constant, then: et = (Rn)/(rwlv) et = (200 W/m2) /(1000kg/m3) (2.5x106 J/kg) = 8.010-8 m/s = 0.7 cm/day However, we need to consider the state (wetness) of the surface to understand and quantify how the received energy is partitioned. Evapotranspiration Estimation of ET 2) from the Energy balance When water is in limited supply, the surface becomes warmer than in the wet cases and more energy is removed from the control volume through conduction in the soil and heating of the air. In this case the surface properties, rather than the atmospheric conditions, are controlling the rate of evapotranspiration. (eg. Higher winds and lower saturation will increase evaporation rate, while reduced solar radiation - clouds - will reduce evaporation) Evapotranspiration Estimation of ET 2) from the Energy balance Relationship between surface wetness and the partitioning of received energy between evaporation and heating of air and soil Evapotranspiration Estimation of ET 2) from the Energy balance The rate of et that occurs under prevailing solar input and atmospheric properties, if the surface is fully wet, is commonly referred as Potential Evapotranspiration (PET). For a catchment water balance, we are interested in the actual et (rate at which water is actually removed). When a surface is wet et/PET = 1, when it is dry et/PET ~ 0 Spatial Variability of Streamflow Large-scale spatial variability in streamflow and storage Spatial Variability of Streamflow Large-scale spatial variability in streamflow is explained by precipitation/evaporation balance to a large extent (~90%) and additional processes to a smaller one (soil water storage, seasonality) Temporal Variability of Streamflow For the Catskill region, the change in evapotranspiration and snowpack amount will offset any increase in precipitation that may occur. Droughts The Concept of Drought Drought is a normal, recurrent feature of climate, although many erroneously consider it a rare and random event. It occurs in virtually all climatic zones, but its characteristics vary significantly from one region to another. Drought is a temporary aberration; it differs from aridity, which is restricted to low rainfall regions and is a permanent feature of climate. Drought, as a normal, recurrent feature of climate, occurs almost everywhere, although its features vary from region to region. Defining drought is therefore difficult; it depends on differences in regions, needs, and disciplinary perspectives. In Libya when annual rainfall is less than 180 mm In Bali after a period of only 6 days without rain! Drought should not be viewed as merely a physical phenomenon or natural event. Its impacts on society result from the interplay between a natural event (less precipitation than expected resulting from natural climatic variability) and the demand people place on water supply. Operational definition of droughts Operational definitions help people identify the beginning, end, and degree of severity of a drought. To determine the beginning of droughts, operational definitions specify the degree of departure from the average of precipitation or some other climatic variable over some time period. This is usually done by comparing the current situation to the historical average, often based on a 30-year period of record. The threshold identified as the beginning of a drought (e.g., 75% of average precipitation over a specified time period) is usually established somewhat arbitrarily, rather than on the basis of its precise relationship to specific impacts. Meteorological drought is usually an expression of precipitation’s departure from normal over time Agricultural drought occurs when there isn’t enough soil moisture to meet the needs of a particular crop Hydrological drought refers to deficiencies in surface and subsurface water supplies Socioeconomic drought occurs when physical water shortage affect supply and demand of economic goods Drought Indices Drought indices assimilate thousands of bits of data on rainfall, snowpack, streamflow, and other water supply indicators into a comprehensible big picture. A drought index value is typically a single number, far more useful than raw data for decision making. Some indices are better suited than others for certain uses. Palmer Drought Severity Index widely used by the U.S. Department of Agriculture to determine when to grant emergency drought assistance (better when working with large areas of uniform topography) Surface Water Supply Index (takes snowpack and other unique conditions into account) useful to supplement Palmer values in Western states, with mountainous terrain and the resulting complex regional microclimates. Drought Indices Palmer Drought Severity Index The PDSI is a meteorological drought index, and it responds to weather conditions that have been abnormally dry or abnormally wet. When conditions change from dry to normal or wet, for example, the drought measured by the PDSI ends without taking into account streamflow, lake and reservoir levels, and other longer-term hydrologic impacts. The PDSI is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil. PDSI is designed for agriculture but does not accurately represent the hydrological impacts resulting from longer droughts. Also, the Palmer Index is applied within the United States but has little acceptance elsewhere. Palmer Classification 4.0 or more 3.0 to 3.99 2.0 to 2.99 1.0 to 1.99 extremely wet very wet moderately wet slightly wet 0.5 to 0.99 0.49 to -0.49 -0.5 to -0.99 -1.0 to -1.99 -2.0 to -2.99 -3.0 to -3.99 -4.0 or less incipient wet spell near normal incipient dry spell mild drought moderate drought severe drought extreme drought Drought Indices Surface Water Supply Index The Surface Water Supply Index (SWSI) was developed to complement the Palmer Index for moisture conditions across the state of Colorado. It is an indicator of surface water conditions and described the index as “mountain water dependent”, in which mountain snowpack is a major component. Like the Palmer Index, the SWSI is centered on zero and has a range between -4.2 and +4.2 (the index is unique to each basin, which limits interbasin comparisons). May 2004 Drought Indices Keetch and Byram Drought Index Keetch and Byram (1968) designed a drought index specifically for fire potential assessment. It is a number representing the net effect of evapotranspiration and precipitation in producing cumulative moisture deficiency in deep duff and upper soil layers. It is a continuous index, relating to the flammability of organic material in the ground. The KBDI attempts to measure the amount of precipitation necessary to return the soil to full field capacity. It is a closed system ranging from 0 to 800 units (800 is the maximum drought that is possible): KBDI = 0 - 200: Soil moisture and large class fuel moistures are high and do not contribute much to fire intensity. Typical of spring dormant season following winter precipitation. KBDI = 200 - 400: Typical of late spring, early growing season. Lower litter and duff layers are drying and beginning to contribute to fire intensity. KBDI = 400 - 600: Typical of late summer, early fall. Lower litter and duff layers actively contribute to fire intensity and will burn actively. KBDI = 600 - 800: Often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels. Drought Monitoring Temporal and spatial distribution Drought forecast using combined models Drought Monitoring Temporal and spatial distribution Drought forecast using combined models Drought Temporal and spatial variability • The Dust Bowl (1933-1940) • The 6-years Texas Drought (1951-1956) Drought Temporal and spatial variability Drought Temporal and spatial variability 12000 2000 1800 10000 1600 1400 8000 1200 6000 1000 800 4000 600 400 2000 200 0 0 May-31 Aug-39 Oct-47 Jan-56 Mar-64 Jun-72 Aug-80 Nov-88 Feb-97 Apr-05 Date Hudson streamflow (m3/s) St. Lawrence streamflow (m3/s) St.Lawrence Hudson North East Atlantic Region Drought Temporal and spatial variability Streamflow forecast and historical data The Colorado River South Texas Reservoirs Nueces River Stream Flow 3.5E+09 3 Rivers Mathis 3.0E+09 Stream Flow (m3/yr) 2.5E+09 2.0E+09 1.5E+09 1.0E+09 5.0E+08 0.0E+00 1910 1920 1930 1940 1950 1960 Year 1970 1980 1990 2000 250000 Volume (AcFt) 200000 South Texas Reservoirs 150000 100000 50000 Series1 0 Nueces River Stream Flow 1/8/56 6/30/61 12/21/66 6/12/72 3 Rivers Mathis 3.0E+09 2.5E+09 Stream Flow (m3/yr) 5/26/83 11/15/88 Year 3.5E+09 2.0E+09 1.5E+09 1.0E+09 5.0E+08 0.0E+00 1910 12/3/77 1920 1930 1940 1950 1960 Year 1970 1980 1990 2000 5/8/94 10/29/99 4/20/05 Drought Temporal and spatial variability http://drought.unl.edu/dm/thumbnails/12_week.gif European Conditions 2003-2005 Annual temperature deviation in Europe in 2003 (Temperature deviation, relative to average temperature from 1961-1990) European Conditions 2005 European Conditions 2005 Precipitation deficit in Spain since beginning of water year (1 Sept, 2004) 97% of Portugal’s territory is affected by “severe” drought conditions