A Detailed Study of Desert Rainfall

Abstract

It is well known that the precipitation that falls during a given rain is not uniform throughout the area covered by the storm. A desire to obtain a better knowledge of these variations in a desert climate and their possible effects on vegetation prompted this study. The only previous investigation of rainfall differences on a small area that has come to my attention is one carried out by Musson ('03) near Windsor, New South Wales. In this work 47 gauges were placed in the form of a cross i mile long and readings were taken after each shower for three months. The highest readings in this series were from 25 per cent to 30 per cent greater than the lowest readings. The author concludes that variations of 20 per cent or more may be expected in the precipitation falling on a single square acre. In August, I930, a series of 24 rain gauges was installed on the grounds of the Desert Laboratory. Sixteen were placed in four rows of four gauges each at intervals of ioo m., thus occupying a square 300 m. on a side. This square was located on the level ground immediately west of Tumamoc Hill. Eight additional gauges were placed in a straight line at i00 m. intervals from the east corner of the square toward the base of Tumamoc Hill and up its slopes to a spot 200 meters above its base (fig. i). A type of gauge was used that consisted of a galvanized iron funnel topped by a vertical collar 5 inches high, inserted through a cork in the top of a quart mason jar. An instrument of this same general type was used by Shreve ('I5) in his studies in the Santa Catalina Mountains of Arizona, and by Cooper ('I7) in a study of precipitation in redwood forests. The gauges used by Cooper had metal rather than glass containers, but were otherwise rather similar. The water was poured from the jar in which it collected into a graduate and this volume was converted into depth of precipitation by dividing the volume ofwater by the area of the mouth of the funnel. The period of observation included all the rains that fell within the year from August 28, I930, to August 28, I93I, and includes 48 periods of precipitation. Readings were made immediately after every rainfall. It may be well to mention briefly the salient features of the rainfall in *the Tucson region. There are two principal periods of precipitation, one usually occurring during the summer months from late June through July and August, and the other falling in the winter months during the period 3 3'