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[1] One potential consequence of global climate change and rapid changes in land use is an increased risk of flooding. Proper understanding of floodwater infiltration thus becomes a crucial component of our preparedness to meet the environmental challenges of projected climate change. In this paper, we present the results of a long-term infiltration experiment performed on fractured ash flow tuff. Water was released from a 3 Â 4 m 2 infiltration plot (divided into 12 square subplots) with a head of $0.04 m, over a period of $800 days. This experiment revealed peculiar infiltration patterns not amenable to current infiltration models, which were originally developed for infiltration into soils over a short duration. In particular, we observed that in part of the infiltration plot, the infiltration rate abruptly increased a few weeks into the infiltration tests. We suggest that these anomalies result from increases in fracture permeability during infiltration, which may be caused by swelling of clay fillings and/or erosion of infill debris. Interaction of the infiltration water with subsurface natural cavities (lithophysal cavities) could also contribute to such anomalies. This paper provides a conceptual model that partly describes the observed infiltration patterns in fractured rock and highlights some of the pitfalls associated with direct extension of soil infiltration models to fractured rock over a long period.

Plate 1. Map of Willcox Basin, Cocliise and Graham Counties, Arizona. Figure 1. Graphs showing fluctuations of water level in typical observation wells, precipitation at Willcox, and pumpage of ground water for irrigation in the ffillcox Basin, Cochise County, Arizona. Purpose and cooperation The Arizona State Legislature has recognized the increasing need in recent years for regulation of groundwater resources in Arizona. Because such regulation must be based upon adequate information as to the quantity, quality, and use, as well as the source and movement of the ground water, the Arizona State Legislature in 1945 appropriated funds for the investigation of the groundwater resources of the State. The investigation is being made by the Geological Survey, United States Department of the Interior, under a cooperative agreement with the Arizona State Land Department, 0. C. Y/illiams, Commissioner. Field work in the "/illcox Basin was started in 1945 and was done by M. B. Booher and H. L. Cushman, engineers, and by H. 3. Jones; geologist, under the general direction of 3. F. Turner, District Engineer (Ground Water) of the Geological Survey, Water analyses were made by J. D. Hem and R. T. Kiser, chemists, under the general direction of C. S. Howard, District Chemist of the Geological Survey. Location and extent The Willcox Basin is a part of a large valley in southeastern Arizona. The northern part of this valley is known as Aravaipa Valley; the central part is referred to in this report as the Willcox Basin; and the southern part is referred to in this report as the Douglas Basin. 15ie Tlillcox and Douglas Basins are commonly known as Sulphur Springs Valley. The ./illcox Basin extends from the drainage divide at the headwaters of Aravaipa Creek southward to the drainage divide among the buttes and ridges near the town of Pearce. Along the eastern side of the Willcox Basin are the Pinaleno, Dos Cabezas, and Chiricahua Mountains; and along the western side of the basin are the Galiuro, Winchester, Little Dragoon, and Dragoon Mountains. The 7/illcox Basin ranges from 10 to 25 miles in width and covers about 1,200 square miles. Although most of the basin is within Cochise County, approximately 250 square miles in the northern part of the basin is within Graham County. CliBBtological data Climatological data obtained at the U. S. Weather Bureau station near ''illcox (elev. 4,200 ft.) are probably representative of climatic conditions in most of the basin. Table 1 contains a summary of precipitation, mean temperature, and evaporation at this station. Precipitation at the station in 1945 was 6.42 inches, and the annual mean temperature in 1945 was 58.8 degrees Fahrenheit. The last killing frost in the spring was on April 17, and the first killing frost in the fall was on October 25, so that there were 191 £rost-free days during the 1945 growing season. History of development Ground water was first obtained in the Y/illcox Basin in 1872 from shallow dug wells for domestic and stock use. About 1905, parts of the basin were se...