On the observed inverse relationship between rainfall amount and dissolved mineral content

Authors: Stout, John

Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2019
Publication Date: 6/26/2019
Citation: Stout, J.E. 2019. On the observed inverse relationship between rainfall amount and dissolved mineral content. Journal of Hydrometeorology. 20:1235-1240. https://doi.org/10.1175/JHM-D-18-0204.1.
DOI: https://doi.org/10.1175/JHM-D-18-0204.1

Interpretive Summary: As the Ogallala Aquifer is gradually depleted, many farms on the high plains of the Llano Estacado will be forced to shift from irrigated to rain-fed agriculture. As the region’s economy becomes increasingly dependent on rainfall, knowledge of various aspects of precipitation becomes increasingly important. Agricultural producers will need to know more about the quantity of rain, the timing and distribution of rain events, and possible variations of rainfall chemistry. In an attempt to study rainfall chemistry, the author has been collecting rainfall samples at two locations on the Llano Estacado for the past five years. The initial focus of this study was to determine if rainwater chemistry tends to vary seasonally or if atmospheric conditions such as wind direction or wind speed had any affect on rainfall chemistry. The analysis of more than four hundred rainfall samples suggest a very weak correlation between rainfall chemistry and any of these atmospheric factors. Further analysis of theses samples; however, reveal a significant inverse relationship between the concentration of total dissolved solids (TDS) and the volume of rainfall collected in the rain gauge. The reason for this inverse relationship is not obvious to the casual observer but results clearly suggest that brief low-magnitude rainfall samples of less than 1 mm typically often contain a higher concentration of dissolved solids compared to larger rain events of many millimeters. A theory was proposed that explains why the concentration of dissolved solids in a collected rainfall sample tends to be inversely proportional to the amount of rainfall.

Technical Abstract: Rainfall samples collected on the Llano Estacado exhibit a high degree of variability with respect to the concentration of dissolved solids. This observed variability has not been fully explained and there remains some uncertainty regarding factors that influence the ionic composition of individual rain samples. Measurements often show a distinct decrease in concentration with increasing precipitation amount. The reason for this inverse relationship is not intuitively obvious, however, it can be explained from a theoretical perspective. An equation was proposed that describes the concentration of dissolved solids in a collected rainfall sample. The theoretical basis of the derived equation rests upon fundamental principles of conservation of fluid volume and conservation of mass. This equation suggests that if the rain sampling tube is absolutely clean at the start of a rain event, then the rainfall sample will not be altered by its collection and, therefore, will provide a true measure of rainfall chemistry. However, if windblown dust or other impurities are allowed to deposit in the rain gauge prior to or during the early stages of a rain event then the concentration of dissolved solids can be very large for small sample volumes and not at all representative of the true concentration within the rain cloud. Results suggest that impurities in the rain sample can be appreciably diluted by the addition of relatively pure rainwater such that the concentration will asymptotically approach the true concentration as the rainfall sample volume increases.