18 Aug 2023
 | 18 Aug 2023
Status: this preprint is currently under review for the journal NHESS.

Agronomic and edaphic drought relations. A semiarid rangeland case

Juan J. Martin-Sotoca, Ernesto Sanz, Antonio Saa-Requejo, Rubén Moratiel, Andrés F. Almeida-Ñauñay, and Ana M. Tarquis

Abstract. The dynamic of rangelands results from complex interactions between vegetation, soil, climate, and human activity. In arid and semiarid areas, rainfall coefficients of variability are over 30 %. This scenario makes rangeland's condition challenging to monitor, and degradation assessment should be carefully considered to study grazing pressures. In the present work, we study the interaction of vegetation and soil moisture in arid rangelands, using vegetation and soil moisture indexes. We aim to study the feasibility of using water soil moisture (soil drought) as a warning index for vegetation drought. An arid agricultural region in the southeast of Spain, in the province of Almeria (Los Vélez), was selected for this study.

MODIS images, with 250 and 500 m spatial resolution, from 2002 to 2019, were acquired to calculate the anomaly (Z-score) for the Vegetation Condition Index (ZVCI) and the Water Condition Index (ZWCI). ZVCI was calculated using the Normalised Difference Vegetation Index (NDVI). Soil moisture component (W) was obtained using the Optical Trapezoid Model (OPTRAM). The probability of coincidence of their negative anomalies between ZVCI and ZWCI, with lags between them, was calculated. The results show that for specific seasons, the anomaly of the water content index had a strong probability of informing in advance where the negative anomaly of VCI will decrease. Soil water content and vegetation indices show more similar dynamics in the months with lower temperatures (from autumn to spring). In these months, given the low temperatures, precipitation leads the vegetation growth. In the following months, water availability depends on evapotranspiration and vegetation type as the temperature rises and the precipitation falls. The stronger relationship between precipitation and vegetation from autumn to the beginning of spring is reflected in the feasibility of ZWCI to aid the prediction of vegetation index anomalies. During these months, using ZWCI and ZVCI as warning indices are possible for two Spanish semiarid rangeland areas: Los Vélez (Almería) and Bajo Aragón (Teruel). Particularly, November to January showed an average increase of 20–30 % in the predictability of vegetation index anomalies. We find other periods of relevant increment in the predictability as March and April for Los Vélez, and July, August and September for Bajo Aragón.

Juan J. Martin-Sotoca et al.

Status: open (until 29 Oct 2023)

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Juan J. Martin-Sotoca et al.

Juan J. Martin-Sotoca et al.


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Short summary
This work includes vegetation (VCI) and water content index (WCI) series from two semiarid rangeland areas in Spain. Based on then, a Z-score for both was calculated to use it as an anomaly index. In this way, we associated negative anomalies with drought episodes. Then, we study the relations of these negative anomalies to see if it is possible to use WCI as an alarm of agronomic drought (VCI negative anomaly). The description of the behaviour of both areas and their comparison are made.