Soil Moisture and Groundwater Recharge Outline v Introduction

Soil Moisture and Groundwater Recharge

Outline v. Introduction v. Porosity and Water Content of Soil v. Capillarity and the Capillary Fringe v. Pore-Water Tension in the Vadose Zone v. Soil Water

Introduction v The uppermost layer of the Earth (zone of aeration, unsaturated zone, vadose zone) contains 3 phase system of solid, liquid, and gaseous material. v Solid phase: mineral grains, organic matter. v Liquid phase: Water containing dissolved solids v Vapor phase: Water vapor and other gases.

Porosity and Water Content of Soil v The porosity of the soil is the percent of void space: v The void ratio of the soil is the ratio of the volume of the voids to the volume of the solids: v The total volume is equal to the volume of the voids plus the volume of the solids: v The void ratio is closely related to the porosity if porosity is expressed as a ratio:

Porosity and Water Content of Soil v The gravimetric water content of the soil is the mass of the contained water divided by the mass of the solid particles (dry mass of the soil): v The volumetric water content of the soil is the volume of the contained water divided by the total volume of the soil: v The saturation ratio of a soil is the volume of the contained water divided by the volume of the voids:

Porosity and Water Content of Soil v The dry bulk density of the soil is the mass of the soil particles (dry mass) divided by the volume of the sample: v The particle density is the mass of the mineral particles of the soil divided by the volume of the soil particles: v To measure soil moisture: (1) indirect method (measure electrical resistance of soil which is proportional to the moisture), (2) Neutron meter (neutron become slow when encounter hydrogen atoms in water).

Capillarity and the Capillary Fringe v Fluid pressure above water table are negative with respect to local atmospheric pressure. This is called Tension. v Water molecules at water-table are subject to an upward attraction due to surface of tension of the airwater interface and the molecular attraction of the liquid and solid phase. This is known as capillarity.

Capillarity and the Capillary Fringe The rise of a fluid in a capillary tube is given by: Rise of fluid due to capillary force. hc is the height of the capillary rise. s is the surface tension of the fluid. l is the angle of the meniscus with the capillary tube. rw is the density of the fluid. g is the acceleration of gravity. R is the radius of the capillary tube.

Pore-Water Tension in the Vadose Zone v Fluid pressure in the unsaturated zone are negative. v The negative pressure head is measured in the field with a tensiometer. Tensiometer

Soil Water v The water existing in the vadose zone is called soil water (for growing plants). This zone extends from land surface to the depth of plant roots (1 -2 meters). v Soil moisture depends on amount of precipitation and evapotranspiration. Soil moisture budget