Flow Routing in a Cratered Landscape: Model Validation and Application to Mars

Fluvial features seen in images from Mars missions indicate that Mars once had water from precipitation flowing across the surface. Like Mars, the Great Basin province in the southwestern United States consists of many enclosed basins that are fed by rivers, which overflowed to downstream basin lake(s) when conditions were permitting. We envision the Great Basin as an analog to the period early in Martian history when precipitation and runoff eroded the valley networks. A spatially-explicit hydrological model was applied to the Great Basin region and to the southern highlands of Mars to predict runoff magnitude and lake distributions. The model iteratively routes runoff through, and evaporation from, depressions to find a steady state solution. For the Great Basin study, the model was calibrated with spatially-explicit annual precipitation estimates and compiled data on pan evaporation, mean annual temperature, and yearly river runoff. The predicted lake distribution provides a close match to present-day lakes. The model was then utilized to estimate differences in annual precipitation and mean annual temperature that replicate the mapped distribution of lakes during late Pleistocene. The sizes of lakes Bonneville and Lahontan were well predicted by linear combinations of decrease in mean annual temperature from 0 to 6 °C and increases in precipitation from 0.8 to 1.9 times the present values. To reproduce Lake Manly in Death Valley, however, combinations of temperature decrease up to 9 °C or precipitation up to 2.4 times the present values were required. Possible explanations for iii this latitudinal difference in estimated climate include groundwater contributions to Lake Manly or spatially non-uniform climate due to topography. Because atmospheric and hydrologic conditions of early Mars are highly uncertain, relative values of evaporation and runoff from precipitation were used to estimate possible climatic conditions of early Mars (> 3.7 Ga). The model predicts the flowpaths and possible lake distributions using assumed ratios of net lake evaporation to runoff. The estimated ratios are consistent with those calculated for the Great Basin lakes. The results suggest rainfall and evaporation on early Mars may have varied regionally.

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