Water Recharge in a Soil with Shrinkage Cracks

Abstract

AbstractThe purpose of this study was to observe flow of water from the soil surface into a relatively dry Pelosol characterized by fine shrinkage cracks between peds. In this soil, for example, a coefficient of linear expansion of 0.115 was measured in peds of the Bva horizon between oven dryness and the 0.3‐bar percentage.After a prolonged period in which evapotranspiration exceeded rainfall, 200‐µCi of tritium were added with 100 liters of water to a plot 1.4 by 1.4‐m. Replicated soil samples were taken after 24‐hours and again 14‐days later.Had tritiated water replaced older water, as may be considered normal in granulated soil, it would have remained in the surface 15 or 20 cm of the profile. In fact it was not confined to surface layers, about half being found below the 20‐cm depth. Small amounts were found to the 100‐cm sampling depth.Water content measurements indicated that the 50‐mm applied tritium water did not wet the surface layers to the 0.3‐bar percentage. Water additions of 765‐mm applied to an adjoining plot over a period of 1 month showed gradual accumulation in the surface horizons of the profile rather than wetting to “field capacity” in layers that proceed with time from the soil surface downward.These results both of tritium and water flow are interpreted to indicate that water flowed down fine cracks in A and B horizons as free water, some of it soaking into the ped walls, or running into the C horizon. This was confirmed by measurements of the tritium concentration on crack walls. It was found to be significantly higher on ped surfaces in the Bva horizon than in soil taken from within the peds.