An approach to the development of isohyetal maps for mountainous areas

Abstract

A technique was developed in which anomalies from precipitation-elevation relationships were used in preparing isohyetal maps of Utah. October–April and May–September precipitation normals (averages for the 1921 to 1950 period) were computed for all available Utah precipitation records. The double-mass analysis technique was used in the derivation of the October–April normals, and for this purpose the state was separated into eight climatic divisions. Relationships between precipitation and water equivalent of snow cover at a network of 29 stations were used in estimating normal October–April precipitation for snow courses located above 8000 feet. The May–September precipitation values for short-term stations were adjusted to the 1921–1950 period by the ratio method. The state was divided into 20 geographic zones, and for these zones it was found that good correlations existed between precipitation and station elevation for both the October–April and May–September periods. The precipitation-elevation relationships showed marked differences for some adjacent areas separated by high mountain ranges. From combined data for several zones, general precipitation-elevation curves for larger areas were obtained and departures of individual station normals from these curves were plotted on a contour base map. Analysis of these anomalies showed that the departures were related to physiographic features. Normal May–September and October–April values were determined for a grid of points over the state by using the anomaly pattern and general precipitation-elevation relationships. These values, together with the observed and adjusted normals, were used to locate the October–April and May–September isohyetals. The large variations in normal precipitation due to topography were taken into account, yet the general precipitation-elevation relationships for small zones were retained. The accuracy of the isohyetal maps is considered comparable to that obtained by presently known methods, and the technique is less time consuming.