Impact of Plant Functional Types on Coherence Between Precipitation and Soil Moisture: A Wavelet Analysis

Abstract

Mapping the spatiotemporal patterns of soil moisture within heterogeneous landscapes is important for resource management and for the understanding of hydrological processes. A critical challenge in this mapping is comparing remotely sensed or in situ observations from areas with different vegetation cover but subject to the same precipitation regime. We address this challenge by wavelet analysis of multiyear observations of soil moisture profiles from adjacent areas with contrasting plant functional types (grassland, woodland, and encroached) and precipitation. The analysis reveals the differing soil moisture patterns and dynamics between plant functional types. The coherence at high-frequency periodicities between precipitation and soil moisture generally decreases with depth but this is much more pronounced under woodland compared to grassland. Wavelet analysis provides new insights on soil moisture dynamics across plant functional types and is useful for assessing differences and similarities in landscapes with heterogeneous vegetation cover.

Multi-Scale Association between Vegetation Growth and Climate in India: A Wavelet Analysis Approach

“…A value of 1 for wavelet coherence indicates a perfect linear correlation between the two signals at the particular time and frequency, while phase difference implies a possible lag between the two signals. Cross wavelet analysis has been employed in applications with non-stationary variables, such as to determine the effect of changing climate on the discharge patterns at continental and regional scales [51,52], understanding rainfall-ground water interactions at different soil depths [53] and the role played by different plant functional types in these interactions [54].…”

Interactive Effect of Meteorological Drought and Vegetation Types on Root Zone Soil Moisture and Runoff in Rangeland Watersheds

“…The normalized WPSs were obtained by dividing by the variance of the time series and were used to identify the dominant scale and time of the series changes [32]. The significance test of WPS was estimated against the red noise spectrum since the soil moisture series has distinctive red noise characteristics [16]. The definition of wavelet coherence (WTC) is similar to that of a correlation coefficient, and the WTC of two time series x n and y n is defined as:…”

“…Considering the edge effect, only the values outside the cone of influence (COI) were used to perform a pointwise significance test (0.05 level) against red noise at each scale. Detailed descriptions of the COI and the significance test of WTC are given in other papers [16,[32][33][34].…”

“…Continuous and excessive depletion of soil moisture in the subsoil layers in the redcedar woodland during a drought episode is likely to be associated with the altered water uptake pattern, the changed rooting structure, and the vegetation phenology of the redcedar, compared to grassland condition [26]. The frequent existence of dry surface layers interrupts the propagation of net precipitation into deeper soil layers and, therefore, affects the coherence of precipitation and soil moisture dynamics [16]. Hydrologically, the depletion of soil moisture also decreases the possibility of the formation of a saturated soil profile, especially during drought years when precipitation is low.…”

“…A shift from pluvial condition to drought intensifies the root zone soil moisture stress and may translate a meteorological drought to a hydrological drought [13,14]. The temporal dynamics of root zone soil moisture are heavily influenced by changes in vegetation functional types [15,16]. The propagation of a precipitation pulse towards soil moisture is attenuated by the vegetation canopy interception and root zone water uptake [17][18][19].…”