# Long Island Groundwater Budget

## Science Center Objects

A groundwater system consists of a mass of water flowing through the pores or cracks below the Earth's surface. This mass of water is in constant motion. Water is constantly added to the system by recharge from precipitation, and water is constantly leaving the system as discharge to surface water and as evapotranspiration. Each groundwater system is unique in that the source and amount of water flowing through the system is dependent upon external factors such as rate of precipitation, location of streams and other surface-water bodies, and rate of evapotranspiration. The one common factor for all groundwater systems, however, is that the total amount of water entering, leaving, and being stored in the system must be conserved. An accounting of all the inflows, outflows, and changes in storage is called a water budget (Alley and others, 1999).

Estimation of the amount of groundwater that is available for use requires consideration of two key elements. First, the use of groundwater and surface water must be evaluated together on a system wide basis. This evaluation includes the amount of water available from changes in groundwater recharge, from changes in groundwater discharge, and from changes in storage for different levels of water consumption. Second, because any use of ground water changes the subsurface and surface environment (that is, the water must come from somewhere), the public should determine the tradeoff between groundwater use and changes to the environment and set a threshold at which the level of change becomes undesirable. This threshold can then be used in conjunction with a system wide analysis of the groundwater and surface-water resources to determine appropriate limits for consumptive use.

System wide hydrologic analyses typically use simulations (that is, computer models) to aid in estimating water availability and the effects of extracting water on the ground-water and surface-water system. Computer models attempt to reproduce the most important features of an actual system with a mathematical representation. If constructed correctly, the model represents the complex relations among the inflows, outflows, changes in storage, movement of water in the system, and possibly other important features. As a mathematical representation of the system, the model can be used to estimate the response of the system to various development options and provide insight into appropriate management strategies. However, a computer model is a simplified representation of the actual system, and the judgment of water-management professionals is required to evaluate model simulation results and plan appropriate actions (Alley and others, 1999).

Estimates of the inflow and outflow into the Long Island groundwater system were evaluated using a compilation of data from 2005-2010 where available. Previous estimates from earlier studies and numerical models are also presented in this section as a comparison to a 2005-2010 index period.

_______________________________