This report calculates that .2% of the water at the surface could be unfrozen as a liquid water brine:

UNFROZEN GROUNDWATER IN THE MARTIAN CRYOSPHERE.
R.E. Grimm, M. Bullock, S.Dec, S. Jepsen, G. Olhoeft, S. Painter, J. Priscu; Dept. of Space Studies, Southwest Research Institute, 1050 Walnut St. #400, Boulder CO 80302 (****@boulder.swri.eduUNFROZEN edu); Dept. of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO; Dept. of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT; Dept. of Geophysics, Colorado School of Mines, Golden, CO; Center for Nuclear Waste Regulatory Analy-sis, Southwest Research Institute, San Antonio, TX.
Lunar and Planetary Science XXXVII (2006) 2090.pdf
"Introduction: Unfrozen water is present as thin films surrounding soil or rock surfaces at subfreezing temperatures. On Mars, this unfrozen water will affect geochemistry and could provide microbial habitats. Unfrozen water may persist kilometers above the base of the cryosphere, or perhaps even transiently at the surface."
...
"The thermodynamic theory of unfrozen soil water [10-11] relates the soil-freezing characteristic curve (temperature vs. unfrozen water content) to the soil-moisture characteristic curve (capillary pressure vs. water content). In this way, parameters derived from the study of unsaturated soils [12] can be used to predict unfrozen water content. Such calculations can greatly extend the predictive range for Mars over em-pirical fits, particularly the effect of freezing-point depression due to briny groundwater."
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2090.pdf