Tufts University Where Will the Water Go? Hydrologic Impacts of Climate Change David Purkey, SEI and Richard M. Vogel Department of Civil and Environmental Engineering Tufts University SEI Climate Change Symposium Tufts University November 30, 2007

Tufts University Previous national water resource assessments were completed years ago: Wollman and Bonem, 1971; Water Resources Council 1968, 1978 National Water Commission, 1973 Methods introduced here apply to local, regional, national and global Climate and Water Assessments Water Availability Is Impacted by Climate, Land Use and Water Use and their Interactions and Changes Background and Motivation I

Tufts University Many recent innovations enable us to perform water resource assessments at extremely fine spatial and temporal scales. Intellectual quest for an analog to the ‘Mach number’ or ‘Reynolds number’ for hydroclimatic systems Background and Motivation II

Background and Motivation III Balancing Water for Humans and Nature by Malin Falkenmark and Johan Rockström 2004

Tufts University Methodology for a National/Global Water Census Many of the following ideas arise from a collaboration with Peter Weiskel (USGS) and others resulting in: Weiskel, P.K., R.M. Vogel, P.A. Steeves, P.J. Zarriello, L.A. DeSimone and K.G. Ries, III, Water-Use regimes: Characterizing direct human interaction with hydrologic systems, Water Resources Research, 43, W04402, 2007 and several other papers in progress.

Tufts University P Traditionally, water availability is defined in terms of NET water balance of a watershed P – ET = SW out * water availability = runoff * reflects both the traditional water-supply perspective, and an aquatic-focused ecological perspective P = Precipitation; ET = Evapotranspiration SW out = Surface-water runoff Assume that GWin = GWout = 0

Tufts University Consider total instead of net water balance… P = SW out + ET * considers both: “green water” (ET) demands of terrestrial ecosystems, including rainfed agriculture, and “blue water” (SWout) demands of aquatic ecosystems and human withdrawals. See Falkenmark and Rockström, 2004

Tufts University From watersheds to hydrologic units … SW in + P = SW out + ET * Considers landscape position, as well as climate. * considers both green and blue water Recent GIS datasets (or gridded models) are essential to this approach: (i.e. National Hydrography Dataset, PRISM Climate Data, etc.) Unit 1 Unit 2

Hydroclimatic Regimes 4 Extreme End-members Arise From Total Water Balance headwater source headwater no-flow terminal flow-through terminal sink P P ET SW + GW ET SW + GW (from Weiskel, Vogel and others., in prep.)

Example from New England Potential Water Availability (= P + SW in ) for each of 308 HUC-12’s of the Conn. River watershed (mean annual) Map by Sara Brandt, using regional hydrologic equations of Vogel and Wilson (1996) Paper on hydroclimatic regimes to appear as Weiskel, Vogel and others, in preparation, 2007

p + (sw in + gw in ) = et + (sw out + gw out ) = 1 - Land-atmosphere fluxes (P, ET) - Landscape fluxes (GW, SW) hydro- system Now, lower case denotes the normalized water balance:

Map of Potential Water Availability for the African Continent From MS Thesis by Sara Freeman Tufts University 2007

Hydroclimatic regime plot Shows relative magnitudes of vertical and horizontal fluxes Deerfield River, MA, HUC-12

Connecticut River basin, hydroclimatic regimes (for 308 HUC-12’s) ET / P Very humid 0 – 0.33 Humid 0.33 – 0.66 Sub-humid 0.66 – 1.0 Semi-arid 1.0 – 1.5 Arid 1.5 – 3.0 Very arid > 3.0 (data compiled by S.. Brandt using Vogel et al regressions) humid very humid sub-humid semi-arid arid very arid = et = p hydroclimatic pathway headwaters mouth

Tufts University Integrating human water use into the water balance … SW in + P + H in = SW out + ET + H out (see Weiskel and others, 2007) H out = withdrawals H in = return flows + imports

… a water balance with three flux classes: - Land-atmosphere fluxes (P, ET) - Landscape fluxes (GW, SW) - Human fluxes (Hin, Hout) A new conceptual model of the terrestrial water balance: hydro- system

Water-use Regimes: 4 end-member (EXTREME) regimes surcharged churned undeveloped depleted Hin Hin Hout SW + GW SW + GW (from Weiskel, Vogel and others 2007) Hout Central Valley Aquifer P ET P - ET

Water-use regime plot Shows relative magnitudes of withdrawals versus return flows and of human vs. natural fluxes. (Weiskel, Vogel and others, 2007)

Tufts University Selected Water-Use Regimes Watersheds From Weiskel, Vogel and others., 2007 Normalized Imports +Return Flows Normalized Withdrawals

Tufts University Selected water-use regimes Aquifers From Weiskel, Vogel and others., 2007 Normalized Return Flows Normalized Withdrawals

Tufts University Seasonal (Monthly) Water Use Regimes Upper Charles River Aquifer, Massachusetts Regimes are sensitive to seasonal climate and water use variations Based on transient simulations of Eggleston (2003) Normalized Withdrawals Normalized Return Flows

Tufts University A Water Resource Development Pathway Mississippi River Alluvial Aquifer, Predevelopment 1918 to 1998 Water use regimes are subject to trends Based on transient simulations of Reed (2003) Normalized Withdrawals Normalized Return Flows

Tufts University Sustainable Water-Use Regimes A rich topic for future research For example relative Net demand RND RND>0.2 implies STRESS Constant RND Normalized Withdrawals Normalized Return Flows

Tufts University Green Water Management Potential Green water management strategies are most attractive in hydrologic units with high water use intensity AND high green water availability

An Indicator of Green Water Management Potential From MS thesis Sarah Freeman Tufts University 2007

Tufts University Traditional focus has been on net water balance of watersheds Focus was on blue- water demands of humans and aquatic ecosystems Traditional water assessments did not fully incorporate humans into the water balance Focus was on watersheds, whereas water availability also depends upon location WITHIN watershed Total water balance of hydrologic units offers a more comprehensive view of hydroclimatology Summary

Tufts University Water Resource Assessments Must Focus on Hydrologic Units (HU’s) and total water balance because: 1- Total water balance focuses on blue and green- water demands of humans (e.g., rainfed agriculture) and terrestrial ecosystems 2-Water is managed in hydrologic units 3- Spatial datasets are gridded which is consistent with HU’s 4-Integrated water balance is needed for full incorporation of humans into water cycle Summary

Tufts University Climate Elasticity of Streamflow Sankarasubramanian, Vogel and Limbrunner, Climate Elasticity of Streamflow in the United States, Water Resources Research, 2001.Climate Elasticity of Streamflow in the United States