Precipitation is the process by which liquid and solid-phase aqueous particles, such as rain, snow, sleet, and hail, fall from the atmosphere to Earth's surface. The occurrence of precipitation over land is typically cited as the driving force of the hydrologie cycle, since it triggers the commencement of other fluxes (évapotranspiration, runoff, infiltration) by providing a new source of moisture to the system. The intensity and frequency of precipitation vary considerably both spatially and temporally, and the effects of precipitation can be both welcome (e.g., during droughts) or undesirable if it occurs in excess and causes subsequent flooding. In some regions, where dry air dominates the weather conditions, precipitation may fall from the clouds but evaporate before ever reaching the ground; this is a phenomenon known as virga. Measurements and estimates of precipitation (volume and intensity) are critical to any study or modeling effort involving the hydrologie cycle. Rain gages have been the primary mechanisms for observation, but their sparse distributions and other limitations do not provide the spatial and temporal resolution needed for various modeling and research efforts. Recent advances are rapidly improving the situation by merging satellite and radar with gage information. Examples of such work are discussed in Smith (Chapter 24), Bales and Cline (Chapter 25), Sorooshian et al. (2000), Adler et al. (1993), Arkin and Xie (1994), and Xie and Arkin (1995, 1996, 1997).


Evaporation is defined as "the rate of liquid water transformation to vapor from open water, bare soil or vegetation with soil beneath" (Shuttleworth, 1993), and transpiration is the rate of water added to the atmosphere as it moves from soil through the stomata of vegetation. Evapotranspiration (ET) is thus a compound term that describes the collective effect of evaporation of water and transpiration of plants. It is the primary process that moves moisture from Earth's surface to the atmosphere. The only other natural means by which water is transferred from the earth to the atmosphere is the process of sublimation, where solid phases of water (e.g., snow and ice) transition directly to atmospheric vapor in the absence of melting. Sublimation typically occurs in regions of cool temperatures and low relative humidity. Evapotranspiration is often an elusive variable to quantify, as it varies diurnally, seasonally, and with changes in precipitation events. A more thorough discussion of evaporation, including a description of various evaporation measurement techniques, may be found in Chapter 26.


Runoff is generally thought of as the movement of excess rainfall across the land surface into rivers, lakes, or the ocean. It occurs when the rate of precipitation exceeds the rate of infiltration at the soil surface, or when soil is saturated. Runoff is a particularly important process at the catchment scale, since it can recharge reservoirs and replenish rivers that may subsequently recharge the groundwater; runoff can also cause soil erosion, and excess runoff can lead to flooding. In Chapter 29, Beven offers a broader historical description of the definition of runoff and also describes various hydrological components that contribute to its generation.


Natural groundwater fluxes are typically slow; water may reside in an aquifer for as little as a few hours (as in the case of river bank storage) or for hundreds of years. Accordingly, groundwater itself is often perceived, on the average, as a relatively slow-moving reservoir in the global hydrologic cycle. At the catchment scale, however, where stream-aquifer interactions are relatively rapid and substantial, the average groundwater fluxes are relatively fast moving. They comprise: (1) the natural flow of water between watersheds, (2) the water pumped from an aquifer, (3) mountain-front recharge (seasonal infiltration of snowmelt at the base of mountain ranges), (4) event-based infiltration (infiltration from precipitation and subsequent rises in surface water levels, especially rivers), and (5) artificial recharge via anthro pogenic conservation projects. To better comprehend such complex hydrologie flow scenarios, it is critical to first understand the basic principles of groundwater flow. In Chapter 28, Yeh not only describes Darcy's law, the fundamental flow equation for fluid in porous media, but also reviews flow equations for various aquifer conditions (e.g., confined, leaky, unconfined) and describes the use of groundwater flow models used for water resources management.

The Water Balance: Global to Catchment Scale

The water balance simply refers to the volumes of water that flow through various components of the hydrologie cycle. More specifically, it is another useful conceptual model in which the components of the hydrologie cycle are evaluated as storage units that are affected by various inputs and outputs. If the various components of the cycle can be quantified or at least estimated, it is possible to gain an understanding of how alteration of a component might affect the balance of the hydrologie cycle. The most simplistic formulation of a water balance is denoted by the elementary continuity equation that conveys the notion that "input to a hydrologie system equals the output from the system, plus or minus any changes in storage" :

where, for a given domain, I is the total inflow, comprised of surface runoff (into the domain), groundwater inflow and precipitation; O is the outflow of évapotranspiration, surface runoff (out of the domain), and groundwater; and AS is the change in storage, whose variables are determined by the scale of the domain.

The concept of a water balance is useful at both global and regional scales. At the basin or watershed scale, where groundwater-surface water interactions might encompass the primary focus, precipitation and groundwater inflow would be a model's input, while overland flow, groundwater outflow, and évapotranspiration

Figure 3 (a) Water balance at the catchment scale, (b) atmospheric water balance, (c) combined land surface atmosphere water balance (after Oki, 1995, 1999).

would be its outputs. Figure 3« shows a conceptual model for the water balance at a watershed scale. Note that the figure does not represent changes in storage caused by anthropogenic activities such as pumping, artificial recharge, or surface water diversions from or to other basins. The consideration of such anthropogenic effects in determining the water balance may be critical, depending on the spatial and temporal scales under consideration.

For meteorologists, the most relevant transfers of water in the hydrological cycle are the vapor flux and moisture exchanges between the atmosphere and Earth. With Earth's surface as the focal point of the cycle, we can evaluate precipitation as the major input to the system, while evaporation and transpiration output moisture to the atmosphere. The change in storage could include, for example, the infiltrated water that is not reevaporated into the atmosphere, or water that becomes frozen in polar ice caps. Such water is temporarily and relatively static in the land surface-atmosphere system. That is, given that atmospheric scientists tend to evaluate the hydro-logic cycle over a time frame of about 8 to 10 days {i.e., the average amount of time that water cycles through the evaporation-condensation-precipitation cycle), water that resides as ice or becomes slow-moving groundwater is seen as a very slow change in storage of the land surface-atmosphere system. Figures 3b and 3c show conceptual models of the water balance in the atmosphere and the combined basin atmosphere water balance, respectively.

In addition to catchment-scale analyses, the evaluation of the water balance of the hydrologic cycle at increasingly larger scales is important because the issues and

Spatial Resolution lssue\

Continental Scale: Focus of climate modelers

Different Scales Different Issues Different Stakeholders

Watershed Scale: Where hydrology happens Where stakeholders exist

Figure 4 Identification of the spatial scales at which hydrologic phenomena are measured. Different scales delineate different stakeholders, and also determine the various levels of water management issues.

stakeholders are different for each scale. Figure 4 illustrates the different stakeholder affiliations at various scales of hydrologic investigation. The uppermost illustration of North America shows outlines of continental-scale basins, and the adjacent text indicates that the corresponding stakeholders arc climatc modelers. The results of climate modelers' research potentially affcct international, global policies and may influence industrial emissions standards for greenhouse gases. In the middle illustration of North America, sub-basins are delineated, and in the bottom illustration, copious individual watersheds are outlined. The sub-basin to watershed scales are where hydrology happens on scales at which most people can observe more immediate and obvious impacts to their local water supply. Water resources management issues at the watershed scale are thus clearly different than those of the sub-basin and continental basin: however, our understanding of the water cycle at all scales—both spatial and temporal—is critically important to addressing the needs of various Stakeholders.

Figure 5 shows how specific water resource issues vary in space and time. The spatial scalc varies from 1CI to 10f' km2, and the temporal scale varies from days to centuries. Across the top of the diagram, the types of prediction are identified that can be made for a corresponding time scale: the shortest predictions are weather forecasts (ranging from less than a day to several days), while the longest predictions are climate change (on the order of centuries). The center of the diagram identifies the types of water resources management issues corresponding with different permutations of spatial and temporal ordinates.

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Figure 5 Schematic illustration of how water resources issues vary across spatial and temporal scales (after National Research Council, 1998).

Tanpvral Scale

Figure 5 Schematic illustration of how water resources issues vary across spatial and temporal scales (after National Research Council, 1998).


Modeling efforts can help us understand the potential impact of human activities on the hydrologic cycle and climate in particular. Unfortunately, our current efforts are hampered by a lack of quantitative data on the distribution and flux of water in its various states, and by our uncertainty of the interactive functioning within the hydroclimatological system (Chahine, 1992). To address these uncertainties, the Global Energy and Water Cycle Experiment (GEWEX) was initiated by the World Climate Research Programme (WCRP) in 1988, to observe and model the hydro-logical cycle and energy fluxes in the atmosphere, at the land surface, and in the upper oceans. A complementary, parallel program, known as the Biospheric Aspects of the Hydrologic Cycle (BAHC), was initialized by the International Geospheric-Biosphere Programme (IGBP) to complement the GEWEX program by placing the emphasis on the biological aspects of the hydrologic cycle—particularly the role of plants in the vertical transfer of water and carbon between the land and atmosphere. For its part, GEWEX has served as a coordinating body of scientists who initiate and facilitate communication among numerous international research teams investigating various aspects of hydrometeorological processes. The hydrological cycle between the land surface and upper atmosphere has subsequently received considerable attention (Chapter 27). Scientists have begun to suggest that we should also consider how land-atmosphere interactions at the basin-scale affect or are affected by climate.

The National Research Council (1998) stated that most water resources management problems are addressed at the sub-basin and watershed scales. Five GEWEX continental-scale experiments (CSEs) have been making promising contributions to improving our understanding of the water balance at scales small enough to be useful for water resources management purposes. For example, the first CSE to be established was GCIP, the GEWEX Continental-Scale International Project, a large-scale study of the Mississippi Basin. During its early phases, GCIP developed data sets, models, and a research framework to better understand and predict land-atmosphere interactions on climatic time scales (seasonal and annual) in the Mississippi River Basin. In fact, GCIP succeeded in meeting most of its objectives, and the project has since transformed to encompass the entire continental United States, as well as part of northern Mexico. This follow-on research project is called the GEWEX America Prediction Project (GAPP). Additional CSEs were selected to represent different climatic conditions than in the Mississippi River Basin. Evaluated together, and separately, the resulting coupling of land surface models with atmosphere and ocean models is a primary step toward improved climate prediction (Chahine, 1992). Such improvements of operational hydrologic and water resources management tools are critical in helping to bring global and GCIP/GEWEX-scale climate predictions down to a scale important for addressing local and regional water resources issues (National Research Council, 1998).

With the ever-increasing popularity of geographic information systems (GIS) and remote sensing (RS), we are witnessing many new advances in hydrologic modeling, particularly distributed models, which more accurately represent spatial features. Engman and Mittikalli (Chapter 35) provide a brief summary of GIS and RS issues.


A stochastic process is described by a randomly determined set of observations, each of which is a sample of one element from a probability distribution. Virtually all hydrologie processes can be characterized as stochastic. It is therefore not surprising that the development and application of statistical and stochastic methods in hydrology date back several decades (e.g., Fiering (1967, 1976); Haan (1997); Chow et al. (1998), among many others). The application of flood frequency analysis in hydro-logic design and operation of water resources systems is a good example of how influential and powerful these methods have become. Valdés et al. (Chapter 34) address the methods used for stochastic forecasting, while Salas et al. (Chapter 33) discuss stochastic simulations in the context of precipitation and streamflow. Forecasts are generally applied to operational and management scenarios, while simulations are used in the context of design and planning. More recently, it has become increasingly popular to apply stochastic simulation tools to more thoroughly address the uncertainties of hydroclimatic processes. Salas and Pielke (Chapter 32) provide an excellent review of the current state of the literature in this area.


The discussion provided above is a brief overview of the various elements of the hydrologic cycle (fluxes and processes) and also offers a summary of ongoing related research activities. It is expected that research and development activities in hydrology and water resources will continue to follow two general paths: theoretical and applied. Theoretical research most related to this handbook will be driven by the need to more accurately close the water budget and quantify the energy cycle at various spatial and temporal scales. As discussed in the chapters that follow, we expect to see future advances in observational tools and applications (e.g., remote sensing, GIS, etc.). We may also expect more advanced modeling of hydrologic processes both at the catchment scale as well as scales that are intended to provide coupling with other components of Earth's systems (i.e., atmosphere, ocean, and biogeochemical processes). On the more applied side, the future requirements for adequate water supplies (quantity and quality) will demand further development of both deterministic and stochastic tools that take advantage of more advanced forms of observational, GIS, and computational techniques. These tools will provide prediction and simulation capabilities for assessing the ramifications of hydroclimatic scenarios (e.g., droughts and floods, regional groundwater depletions, existence and movement of contaminants in both surface water and groundwater, etc.).


We gratefully acknowledge the support provided by SAHRA (Sustainability of

Semi-Arid Hydrology and Riparian Areas), an NSF Science and Technology

Center at the University of Arizona, as well as the Global Energy and Water

Cycle Experiment (GEWEX). Our sincere gratitude is also extended to Terri

Hogue, Thomas Pagano, and Corrie Thies, for their thoughtful comments and editing of this document in its various stages of completion.


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