Document PIHM manual calibration

"Manual calibration" of PIHM is a semi-automated procedure consists of 3 steps: calibration of parameters controlling lateral groundwater flow, calibration of parameters controlling infiltration, and offline calibration of a simple temperature index model for snow accumulation melt. These steps are summarized below.

Parameters controlling lateral groundwater flow

Parameters controlling lateral groundwater flow include the horizontal saturated hydraulic conductivity, KsatH, horizontal macropore hydraulic conductivity, MacKsatH, and macropore depth, MacD. The soil thickness, which groundwater flow is also sensitive, is held at an assumed value/distribution. Models are run forward from an initial state of completely saturated soil using all meteorological forcings (with precipitation set to zero) to capture the steepening effect of ET on the slope of the recession curve. Errors are examined between modeled stream discharge (base flow) and observed. The "observed" stream discharge is taken to be the time-dependent solution to the power law -dQ/dt = a*Q^b that best fits observation. Figure 1 (right) illustrates how the different parameters impact the shape of the modeled recession curve.

Examples of of PIHM-modeled versus observed (from power law) stream discharge

The model calibration runs are executed using in-house Python scripts, and consist of all combinations of the above three parameters divided into 10 divisions (1000 runs per set). Generally two sets are carried out per calibration, yielding 2000 runs per calibration.