Preferential transport of water and 131 Iodide in a clay loam assessed with TDR-techniques and boundary-layer flow theory

Abstract

Abstract. Rapid soil moisture variations were measured with TDR equipment at five depths ranging from 0.1 to 0.9 m during five consecutive infiltration experiments under ponding. Each time, 27 mm of water were applied. The water of the second experiment was spiked with 200 mbq of K131I-tracer. Its activity was recorded as functions of depth and time with Geiger-Müller probes in 12 vertically installed access tubes. The soil moisture variations were classified as showing (i) no reaction, (ii) monotonous increase, and (iii) rapid increase followed by a gradual decrease. Reaction type (iii) was investigated further according to the boundary-layer flow theory and diagnosed as preferential flow. Rapid variations of 131I-activities occurred at all depths showing soil moisture reaction type (iii). However, some of the reaction types (i) and (ii) also included rapid variations of the activities. The approach based on boundary-laver flow theory allows fluxes to be estimated from soil moisture variations. Seven estimated total volumes of rapid flow ranged from 0.15 to 1.1 of the applied volume of water, and in only one case was the total volume badly overestimated by a factor of almost 3. The approach is worth further exploration.