Abstract
AbstractThe main source of soil moisture variability in savanna ecosystems is pulsed rainfall. Rainfall pulsing impacts water‐stress durations, soil moisture switching between wet‐to‐dry and dry‐to‐wet states, and soil moisture spectra as well as derived measures from it such as soil moisture memory. Rainfall pulsing is also responsible for rapid changes in grassland leaf area and concomitant changes in evapotranspirational (ET) losses, which then impact soil moisture variability. With the use of a hierarchy of models and soil moisture measurements, temporal variability in root‐zone soil moisture and water‐stress periods are analysed at four African sites ranging from grass to miombo savannas. The normalized difference vegetation index (NDVI) and potential ET (PET)‐adjusted ET model predict memory timescale and dry persistence in agreement with measurements. The model comparisons demonstrate that dry persistence and mean annual dry periods must account for seasonal and interannual changes in maximum ET represented by NDVI and to a lesser extent PET. Interestingly, the precipitation intensity and soil moisture memory were linearly related across three savannas with ET/infiltration ∼ 1.0. This relation and the variability of length and timing of dry periods are also discussed.
Highlights
In savanna ecosystems, sparsely spaced woody vegetation allows ample photosynthetically active radiation (PAR) to reach the ground surface, thereby promoting a herbaceous layer to compete for water with the woody vegetation
Through hierarchy of approximations applied to a lumped water balance model and analysis of measured soil moisture, it was demonstrated that precipitation variability alone explains much of the soil moisture variance at high frequencies
Adjustments to maximum ET loss under water stress (ETs) with normalized difference vegetation index (NDVI) and potential ET (PET) variability improved the model fit to measured soil moisture and concomitant memory timescale estimates. This improvement is of significance when dry persistence and mean annual dry periods are to be estimated from lumped models widely used in climate and ecohydrological sciences
Summary
Sparsely spaced woody vegetation allows ample photosynthetically active radiation (PAR) to reach the ground surface, thereby promoting a herbaceous layer (primarily grasses) to compete for water with the woody vegetation. Soil moisture variability has been shown to exert control on drought occurrence (Masih, Maskey, Mussá, & Trambauer, 2014; Saini, Wang, & Pal, 2016), probabilistic drought prediction (AghaKouchak, 2015), convective rainfall formation (Green et al, 2017; Koster et al, 2004; Siqueira, Katul, & Porporato, 2009; Taylor et al, 2011; Wei, Dickinson, & Chen, 2008) and ecosystem resilience (Porporato, Daly, & Rodriguez-Iturbe, 2004) primarily because soil moisture memory and persistence within certain phases (wet or dry) exceed the timescale of many meteorological variables. The controls on soil moisture memory and dry persistence experienced by African savannas are receiving renewed interest, especially in climate studies (Ghannam et al, 2016; Nakai et al, 2014)
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