Innovative Control Systems for Stable Water Withdrawals and Streamflow Maintenance

Under the water withdrawal regulations and water rights based on a concept of sharing, allowable withdrawals by individual abstractors vary directly with streamflow. The day-to-day administration of such systems, therefore, requires that withdrawal rates be tied to observed streamflows. For many water users, whose reference stream gauge is downstream from the point of abstraction, this poses the possibility of time delay between water withdrawal and streamflow measurement. This in turn induces the possibility of unstable operation of stream water withdrawal systems such that, for example, with a one-day time lag in the stream, the allowable withdrawal may be high one day, resulting in streamflow subceeding the standard the next day, resulting in withdrawal curtailment orders the following day, resulting in rebounding of the streamflow the following day, etc. Such instability exacerbates violations of the streamflow standard and inconvenience in implementation of water withdrawal regulation. Thus resolution of this problem is very significant for successful water withdrawal regulations. This paper explores the application of process control theory to resolve the instability problem through a hypothetical example, in pursuit of meeting minimum in- stream flow requirement and withdrawal demands of abstractors. Since feedback control systems adopted here do not require great accuracy in the model connecting stimulus and response, difficulties in modeling time delay may be overcome to a great extent by a specification of feedback controllers. This paper reveals that with this control technique, we are able to maintain a stable streamflow that meets the minimum in-stream flow standard. This result promises an improvement of the overall reliability of the water withdrawal system and convenient implementation of the withdrawal scheme.