some offer the rationale that green water cannot be captured and moved to a higher valued use in the same way that water in a river aquifer or canal can be delivered to one or more potential users some suggest also that surface water and groundwater often are used to produce higher valued crops while soil moisture often is used to produce lower valued grains and pasture at least four points seem pertinent there is no fundamental principle suggesting that the opportunity cost of soil moisture is always less than the opportunity cost of surface water or groundwater the incremental value of soil moisture can be quite high in many rainfed situations the incremental value of surface water or groundwater can be quite low in many irrigated settings and relative opportunity costs comparative advantages rather than one s own opportunity costs must be considered to determine optimal trading strategies