Chapter summary

Soils store a considerable amount of heat and water. The diurnal cycle of soil temperature and variation in soil temperature over the course of a year are important determinants of the land surface climate. The amount of water held in soil is a key controller of evapotranspiration. This chapter reviews the physics of soil heat transfer and soil water relations. Heat flows from high to low temperature through conduction. Important soil properties that determine heat transfer are thermal conductivity and heat capacity. Two forces govern water movement in soil. Gravitational potential represents water movement due to the force of gravity. The second force, called matric potential, occurs because water is bound to soil particles. Water flows from high to low potential as described by Darcy's law. The Richards equation combines Darcy's law with principles of water conservation to describe the change in soil water content over time. Key hydraulic properties are porosity, matric potential, and hydraulic conductivity. These latter two properties vary with soil water. Soils differ in hydraulic properties in relation to the size and arrangement of pores. The pores in sandy soil are large, water loosely adheres to soil particles, water movement is rapid, and the soil drains rapidly. Pores are smaller in clay soil, water is tightly bound to soil particles, movement is slow, and drainage is impeded. Loams are intermediate, draining more slowly than sands and retaining more water.