Here I argued that bed geometry is the most important thing we need to know for glacier modelling, but a close second is certainly how the glacier slides over its bed (or perhaps if the bed itself is oozing downhill). A key component in answering that question is knowing what the water pressure is down there. If there’s no water pressure at all, then the full weight of the ice is jammed down onto every little nubbin and sliding is very difficult. If the ice is just about ready to float, then sliding is relatively easy. Unfortunately, we lack a definitive means to predict this variable.
This is not to say that people aren’t trying. Particularly in recent years, some very interesting models have been suggested for simulating the system of cavities and conduits that make up the subglacial hydrologic system (see here and here for example). While these models are interesting, however, they are more or less unvalidated: it’s not really possible to say whether they’re right or not, particularly because they depend on a large number of unknown parameters that depend on obtuse concepts such as “characteristic height of bedrock bumps” and “average ice macroporosity.”
In this paper, we try to take a step back and ask whether we can learn anything about these obtuse parameters using a very simple model trying to simulate some very simple data. In particular we try to see what combination of parameters causes the model to produce velocities and discharge at the glacier outlet that look something like what was measured. The glacier in question, by the way, is Kennicott Glacier in Wrangell-St. Elias National Park:
Once again, we use Bayesian statistics. We simulate a whole bunch of potential glaciers, keep the most probable ones, and develop a statistical understanding of the parameters governing the model based on the runs we kept. In this way, we an say things about the subglacial hydrologic system like: 1. uncertainty in how water melts the overlying ice is the chief driver of uncertainty in figuring out the water pressure, and 2. significant water needs to be stored in the overlying ice.