Global Water: A Hydrologist’s View Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington Seattle, WA for SIS 490/590, “Water and security in the Middle East” October 6, 2008

Outline of this talk The global (and regional) water and energy cycles Human needs for potable water Water and food Water development The hydrologic underpinnings Water and climate – the Colorado River basin as an example of climatic sensitivities Conclusions

1. The global (and regional) water and energy cycles

Source: NRC 1975

From Bras, 1990

From Hartmann, 1994 (units in cm/yr)

Annual Water Balance for Major Continental Land Areas

Surface Area and Annual Runoff Volume of Major Continents

Water balance of major global rivers Source: Kooiti Masuda, 2002

Water balance of major global rivers Source: Kooiti Masuda, 2002

Manipulation of the global water cycle – water withdrawals

2. Human needs for potable water

Domestic consumptive use (U.S.) is ~200-250 liters/day Compare with drinking water requirement (about 5 l/day). U.S. domestic consumption has declined slightly over the last two decades. Much of difference between potable water requirement and use is sanitation, laundry, etc. Industrial requirement in developed world is of same order as domestic Total water withdrawals are about 6000 km3/yr Compare with global (land) precip ~150,000 km3/yr (or global runoff ~0.4 x runoff)

Table courtesy Peter Gleick

Table courtesy Peter Gleick

Table courtesy Peter Gleick

3. Water and food

Blue and Green water (after Falkenmark) Green Water is rainfall that is stored in the soil and available to plants. Globally, it makes up some 65 per cent of fresh water resources. It is the basis of rain-fed farming and all terrestrial ecosystems. Runoff, stream base flow and groundwater constitute blue water. Green water may be used only in situ: whereas blue water may be transported and used elsewhere – for irrigation, urban and industrial use, and as environmental flow in streams. Courtesy Wageningen University

Figure courtesy of world soil information, Wageningen University

Figure courtesy of world soil information, Wageningen University

Notes Rain-fed agriculture contributes most of the world’s farm production: 95 per cent in Sub-Saharan Africa where it makes use of only 15-30 per cent of rainfall, the rest is lost, mostly as destructive runoff; The partitioning of rainwater is a dynamic process (governed by rainfall intensity, terrain, land cover and soil) that may be controlled by management of land cover, micro topography and soil conditions; Soils process several times more water than they retain; while soil erosion by runoff and bank erosion by peak flows contribute nearly all the sediment load of streams, leading to the siltation of reservoirs and water courses. This means that management of green water is also management of blue water; Finally, agricultural demand for water is in competition or, even, conflict with the needs of industry, urban populations and the environment. Courtesy Wageningen University

Global Runoff & Water use (http://hydro.iis.u-tokyo.ac.jp/GW/result)

4. Water development

13,382dams, Global Reservoir Database Location (lat./lon.), Storage capacity, Area of water surface, Purpose of dam, Year of construction, … 13,382dams, これは構築したデータベースを用いて、ダムの経年変化を示したものです。 これより、欧米を中心に次第にアジアへと広まっていることがわかります。

Reservoir construction has slowed. The focus on new “infrastructure” is slowing due to economic, political, and environmental factors. All reservoirs larger than 0.1 km3

Global Water System Project IGBP – IHDP – WCRP - Diversitas

Global Water System Project IGBP – IHDP – WCRP - Diversitas Human modification of hydrological systems

Visual from Palmieri, NAS Sackler symposium, 2004

5. The hydrologic underpinnings

Runoff generation mechanisms 1) Infiltration excess – precipitation rate exceeds local (vertical) hydraulic conductivity -- typically occurs over low permeability surfaces, e.g., arid areas with soil crusting, frozen soils 2) Saturation excess – “fast” runoff response over saturated areas, which are dynamic during storms and seasonally (defined by interception of the water table with the surface)

Infiltration excess flow (source: Dunne and Leopold)

Lumped Conceptual (Processes parameterized) Explicit Representation of Downslope Moisture Redistribution Lumped Conceptual (Processes parameterized)

Snow Accumulation and ablation processes

Some characteristics of the hydrology of arid regions Low runoff ratios (~15% for Colorado basin) Highly variable (due both to precipitation variability, and amplification by hydrological processes) Where strong topographic variability is present, highly concentrated runoff production (headwaters may not be arid) Seasonality of evaporative demand in mid-latitude regions amplifies spatial and temporal variations (and role of snow, if present) Groundwater may be more disconnected from surface water than in humid and semi-humid regions

Censored spatial distribution of annual runoff, Colorado River basin (~75% of runoff is derived from 25% of area)

6. Water and climate – the Colorado River basin as an arid regions analog

Western U.S. regional study GCM grid mesh over western U.S. (NCAR/DOE Parallel Climate Model at ~ 2.8 degrees lat-long)

Timeseries Annual Average PCM Projected Colorado R. Temperature Timeseries Annual Average ctrl. avg. hist. avg. Period 1 2010-2039 Period 2 2040-2069 Period 3 2070-2098

Timeseries Annual Average PCM Projected Colorado R. Precipitation Timeseries Annual Average hist. avg. ctrl. avg. Period 1 2010-2039 Period 2 2040-2069 Period 3 2070-2098

Annual Average Hydrograph Simulated Historic (1950-1999) Period 1 (2010-2039) Control (static 1995 climate) Period 2 (2040-2069) Period 3 (2070-2098)

April 1 Snow Water Equivalent

Total Basin Storage

Annual Releases to the Lower Basin target release

Annual Releases to Mexico target release

Magnitude and Consistency of Model-Projected Changes in Annual Runoff by Water Resources Region, 2041-2060 Median change in annual runoff from 24 numerical experiments (color scale) and fraction of 24 experiments producing common direction of change (inset numerical values). +25% +10% +5% +2% -2% -5% -10% -25% Decrease Increase (After Milly, P.C.D., K.A. Dunne, A.V. Vecchia, Global pattern of trends in streamflow and water availability in a changing climate, Nature, 438, 347-350, 2005.) 96% 75% 67% 62% 87% 71% 58% 100% Slide 1 shows a subset of the information on slide 2. The notes are the same for both slides. The Water Resources Regions are colored according to the projected percent change in mean annual runoff for the period 2041-2060, relative to the period 1901-1970. These projections are model-estimated changes associated with hypothetical ("SRES A1B" scenario) changes in climate forcing. The printed value inside each region of the map (slide 2 only) is the majority percentage of the 24 experiments that are in agreement on the direction (increase or decrease) of change; for example, the value of 67 in the Texas Gulf region indicates that 67% (i.e., 16) of the 24 experiments projected a decrease in runoff. Actual future changes in runoff can be expected to differ from these projections, primarily because of departures of actual forcing from the SRES A1B scenario, errors in the models' representation of runoff response to climate forcing, and unforced variability ("randomness") of the climate system. The majority percentages (slide 2 only) should not be read as probabilities, but rather as a combined measure of two factors: the degree of agreement among models and the modeled strength of the forced runoff change relative to modeled internal variability of the climate system. In Water Resources Regions for which a strong majority of experiments agree on the direction of change (Alaska, Upper Colorado, Lower Colorado, and Great Basin), the models suggest that the forced runoff change by 2041-2060 will be large compared to unforced runoff variability. In other Regions, either (1) the forced runoff change will not be large relative to the model estimate of unforced variability, or (2) the forced runoff change will be large relative to unforced variability, but this fact is obscured by substantial differences in model errors from one model to the next. The color of a Region is determined only by changes in runoff produced inside the Region. However, where a downstream Region (e.g., the Lower Colorado or the Lower Mississippi) receives streamflow from one or more upstream Regions, the streamflow through the downstream Region will be affected by runoff changes in both the downstream and the upstream Regions. Thus, increasing runoff in the Upper Mississippi, Ohio, and Tennessee Regions implies increasing flow of the Mississippi River through the Lower Mississippi Region, even though the projected runoff change in the Lower Mississippi Region is small. The figure is based on figure 4 of Milly et al. (2005); that reference documents the computations in detail. The computational differences from the published figures are (1) the geographic scope here is limited to the United States; (2) instead of depicting changes in point values of runoff, this figure depicts only changes in areal averages of runoff over Water Resources Regions of the U.S. Water Resources Council; (3) the composite across experiments is formed from the median instead of the mean. The projected changes are median values over a set of 24 climate-model experiments conducted on 12 climate models. The number of experiments exceeds the number of models, because the experiment was run more than once on some models. The 12 models used were the subset of 23 (IPCC AR4) candidate models that best reproduced the global pattern of observed time-mean streamflow during the 20th Century. Reference: Milly, P.C.D., K.A. Dunne, and A.V. Vecchia, 2005, Global pattern of trends in streamflow and water availability in a changing climate, Nature, v. 438, p. 347-350.

from Seager et al, Science, 2007

Inferred runoff elasticities wrt precipitation for major Colorado River tributaries, using method of Sankarasubramanian and Vogel (2001) Visual courtesy Hugo Hidalgo, Scripps Institution of Oceanography

Unconditional histograms of 1/8 degree grid cell precipitation elasticities from model runs for 20 years, ~1985-2005 VIC NOAH SAC

Summary of precipitation elasticities and temperatures sensitivities for Colorado River at Lees Ferry for VIC, NOAH, and SAC models Model Precipitation-Elasticity Temp-sensitivity (Tmin & Tmax ) %/ 0C Temp-sensitivity ( Tmax) %/ 0C Flow @ Lees Ferry (MACF) VIC 1.9 -2.2 -3.3 15.43 NOAH 1.81 -2.85 -3.93 17.43 SAC 1.77 -2.65 -4.10 15.76

VIC Precipitation elasticity histograms, all grid cells and 25% of grid cells producing most (~73%) of runoff

Composite seasonal water cycle, by quartile of the runoff elasticity distribution

Temperature sensitivity (equal change in Tmin and Tmax) histograms, all grid cells and 25% of grid cells producing most (~73%) of runoff

Spatial distribution of temperature sensitivities (equal changes in Tmin and Tmax) Censored spatial distribution of annual runoff

7. Summary – some key points Coupling of the global energy and water cycles (solar radiation is the driver) Human consumptive use of water is about 1/10 of runoff globally Blue vs green water, and the role of irrigation Dams and the implications of water management Runoff generaiton – saturation vs infiltration excess Climatic sensitivity of hydrology and water resources – possible implications of shift poleward of sub-tropical dry zones