The Stabilization Value of Groundwater and Conjunctive Water Management under Uncertainty

T importance of managing ground and surface water conjunctively increases with water scarcity and with inter and intratemporal fluctuations in precipitations. Both factors are becoming critical in many parts of the world; the former due to increased water demand associated with economic and population growth and the latter due to climate change. A conjunctive ground and surface water system appears in a number of forms, which differ according to the water sources. Surface water may consist of stream flows emanating from aquifers, surface reservoirs or lakes, snowmelt, rainfall or any combination. It may be stable or stochastically fluctuate over time. Groundwater sources—aquifers—may be nonreplenishable or replenishable, deep or shallow, confined or unconfined. The two cases in which only surface water or only groundwater is used lie on both ends of the conjunctive spectrum; these extreme cases occur when one source is always cheaper than the other (scarcity cost included). Conjunctive systems, viewed in this larger context, characterize most irrigation systems worldwide. The term conjunctive signifies that the ground and surface water sources are two components of one system and should be managed as such. Oscar Burt pioneered the analysis of conjunctive water systems more than 40 years ago. More recently, Tsur (1990, 1997); Tsur and Graham-Tomasi; Provencher and Burt; and Knapp and Olson extended the theory to account for stochastic, dynamic, and multiaquifer considerations. The underlying idea is simple. Surface water sources derived from rainfall and snowmelt typically fluctuate randomly from year to year and within a year. Groundwater stocks, on