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Chapter E . 2 Predictability and Uncertainty

It is appropriate to consider water quantity and water quality as two facets of water resources. Both facets are intimately connected to the hydrological cycle, which itself is a component of the Earth's climate system. Since human activities and health are so connected to water resources, it is essential to determine how far into the future we can predict the condition of the water resources of a region with a sufficient level of confidence. When predictions are not possible, resilience must be built into a water system so that human (and natural) needs are not negatively affected. Even when skillful predictions are possible, they are seldom completely accurate. Thus uncertainty needs to be included when scientific analyses and predictions are used for water resource planning and management, particularly for issues such as adaptation or risk management. The prediction of weather and climate are essential aspects of planning for water resources in changing environmental conditions. Lorenz (1979) proposed the concept of forced and free variations of weather and climate. He refers to forced variations as those caused by external conditions, such as changes in solar irradiance. Volcanic aerosols also cause forced variations. He refers to free variations as those which " are generally assumed to take place independently of any changes in external conditions ". Day-today weather variation is presented as an example of free variations. He also suggests that " free climatic variations in which the underlying surface plays an essential role may therefore be physically possible ". However, if the ocean surface and/or land-surface changes over the same time period as the atmosphere changes, then the non-linear feedbacks (i.e. two-way fluxes) between the air, land and water, eliminate an interpretation of the ocean-atmosphere and land-atmosphere interfaces as boundaries. Rather than " boundaries " , these interfaces become interactive media (Pielke 1998a, 2001). The two-way fluxes that occur between the atmosphere and ocean, and the atmosphere and the land-surface (as detailed in Part A of this book), must therefore necessarily be considered as part of the predictive system. On the time scale of what we typically call short-term weather prediction (days), important feedbacks include biophysical (e.g. vegetation controls on the Bowen ratio), snow cover, clouds (e.g. in their effect on the surface energy budget), and precipitation (e.g. which changes the soil moisture) processes. This time scale is already considered to be an initial value problem (Sivillo et al. 1997) since …