BLACK-BOX MODELING OF THE SUBGLACIAL WATER-SYSTEM

Measurements of water pressure beneath Trapridge Glacier, Yukon Territ ory, Canada, yield the following generalizations about subglacial cond itions in the studied region: (1) Even over short distances the subgla cial water system is highly heterogeneous. (2) The subglacial water sy stem consists of at least two distinct components which we refer to as the ''connected'' and ''unconnected'' water systems. (3) Regions of t he glacier bed can switch back and forth from being part of the connec ted or part of the unconnected water system. (4) Large spatial pressur e gradients can exist within the unconnected water system, and between the connected and unconnected systems. (5) Rapid pressure variations can occur in the unconnected water system. (6) Pressure variations in the unconnected water system do not match those in the connected syste m and can, in fact, be strongly anticorrelated with pressure variation s in the connected system. If the water pressure variations in the con nected system are viewed as a forcing and those in the unconnected sys tem as a response to this forcing, the input-output relation between f orcing and response can be efficiently represented as a low-order nonl inear ordinary differential equation. The response of the unconnected system to forcing from the connected system is governed by time consta nts having approximate magnitudes of similar to 1.7 hours and similar to 7.4 hours that we believe are associated with process rates for sub strate compression and pore water diffusion, respectively.