In the case of downstream or transboundary surface water obligations, the intent of regulation is generally to reduce pumping in order to limit cumulative stream depletion over a fixed interval such as a year or multiple years. In the latter case, regulations are intended to maintain minimum streamflow requirements throughout the year. Thus, the impact of ongoing pumping on streamflow needs to consider the pumping history rather than just the pumping that is occurring in the current period. The total stream impact from groundwater pumping at any time after the start of pumping is then equal to the sum of lagged impacts occurring at that time from all pumping that occurred at or before this time, accounting for well- specific distances from the stream.