Ecohydrology of Water-Controlled Ecosystems

The frequency and duration of excursions of the soil moisture process above and below some levels directly related to the physiological dynamics of plants are of crucial importance for ecohydrology. This chapter focuses on the crossing properties of specific levels of soil moisture that are important for vegetation. After a general mathematical derivation of the crossing characteristics typical of shot noise processes, attention is turned to the impact of climate, soil, and vegetation on the duration and frequency of soil moisture excursions below the wilting point, sw, and below the point where transpiration is reduced, s*. These properties will be basic for the study of plant water stress in Chapter 4.

The analysis of the crossing properties is also important for gaining insights into the transient dynamics of soil moisture, either at the beginning of the growing season, to evaluate the time to reach the stress levels after the soil winter storage is depleted, or after a drought, to estimate the time to recover from a situation of intense water stress. The expressions of the mean time to reach s* from field capacity, sfc, will be used in Chapter 7 to extend the definition of water stress to transient conditions and furthermore to analyze different strategies of water use when the winter water recharge is important. This chapter ends with the crossing analysis of the minimalistic models of soil moisture dynamics described in Section 2.6.2.

Although the results of this chapter are extensively used later, an understanding of the mathematical derivations presented in Section 3.1 is not necessary for their use.