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We investigated the water use of Caatinga vegetation, the largest seasonally dry forest in South America. We identified and analysed the environmental phenological drivers in woody species and their relationship with transpiration. To monitor the phenological evolution, we used remote sensing indices at different spatial and temporal scales: normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), and green chromatic coordinate (GCC). To represent the phenology, we used the GCC extracted from in-situ automated digital camera images; indices calculated based on sensors included NDVI, SAVI and GCC from Sentinel-2A and B satellites images, and NDVI products MYD13Q1 and MOD13Q1 from a moderate-resolution imaging spectroradiometer (MODIS). Environmental drivers included continuously monitored rainfall, air temperature, soil moisture, net radiation, and vapour pressure deficit. To monitor soil water status and vegetation water use, we installed soil moisture sensors along three soil profiles and sap flow sensors for five plant species. Our study demonstrated that the near-surface GCC data played an important role in permitting individual monitoring of species, whereas the species’ sap flow data correlated better with NDVI, SAVI, and GCC than with species’ near-surface GCC. The wood density appeared to affect the transpiration cessation times in the dry season, given that species with the lowest wood density reach negligible values of transpiration earlier in the season than those with high woody density. Our results show that soil water availability was the main limiting factor for transpiration during more than 80% of the year, and that both the phenological response and water use are directly related to water availability when relative saturation of the soil profile fell below 0.25.

Projected changes in rainfall seasonality and interannual variability are expected to have severe impacts on arid and semi‐arid tropical vegetation, which is characterized by a fine‐tuned adaptation to extreme rainfall seasonality. To study the response of these ecosystems and the related changes in hydrological processes to changes in the amount and seasonality of rainfall, we focused on the caatinga biome, the typical seasonally dry forest in semi‐arid Northeast Brazil. We selected four sites across a gradient of rainfall amount and seasonality and analysed daily rainfall and biweekly Normalized Difference Vegetation Index (NDVI) data for hydrological years 2000 to 2014. Rainfall seasonal and interannual statistics were characterized by recently proposed metrics describing duration, timing and intensity of the wet season and compared to similar metrics of NDVI time series. The results show that the caatinga tends to have a more stable response with longer and less variable growing seasons (3.1 ± 0.1 months) compared to the duration wet seasons (2.0 ± 0.5 months). The ecosystem ability to buffer the interannual variability of rainfall is also evidenced by the stability in the timing of the growing season compared to the wet season, which results in variable delays (ranging from 0 to 2 months) between the peak of the rainfall season and the production of leaves by the ecosystem. The analyses show that the shape and size of the related hysteresis loops in the rainfall–NDVI relations are linked to the buffering effects of soil moisture and plant growth dynamics. Finally, model projections of vegetation response to different rainfall scenarios reveal the existence of a maximum in ecosystem productivity at intermediate levels of rainfall seasonality, suggesting a possible trade‐off in the effects of intensity (i.e. amount) and duration of the wet season on vegetation growth and related soil moisture dynamics and transpiration rates. Copyright © 2016 John Wiley & Sons, Ltd.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

R E S U M OPropôs, neste trabalho, avaliar a distribuição e a interação entre os componentes do balanço hídrico em uma área de pastagem cultivada em Serra Talhada, no semiárido pernambucano. Foram realizados monitoramentos da precipitação pluvial (P), da variação no armazenamento de água (∆A) e do escoamento superficial (ES) para determinação do balanço hídrico. A evapotranspiração (ET) foi obtida a partir da covariância dos vórtices turbulentos. O fluxo total de água no solo (Q), ou seja, a drenagem (-Q) ou ascensão capilar (+Q) foi obtido a partir da equação do balanço hídrico. A taxa de ET da pastagem foi baixa; ainda assim, foi o principal componente de saída de água do sistema seguido pelo ES que, juntos, totalizaram aproximadamente 80% da precipitação pluvial. Esta pesquisa demonstrou que mesmo em anos de seca é necessário se quantificar o ES e as perdas e/ou ganhos de água no limite inferior do solo para se ter uma avaliação precisa da dinâmica da água no sistema solo-pastagem em condições de semiárido.Water balance in pasture area in semi-arid region of Pernambuco A B S T R A C TThis study aimed to evaluate the distribution and the interaction between the components of the water balance in pasture area cultivated in Serra Talhada, in the semi-arid region of Pernambuco. Precipitation (P), the variation in water storage (∆WS) and runoff (R) were monitored for determination of water balance. The evapotranspiration (ET) was obtained from the Eddy covariance method. The total flow of water in the soil (Q), i.e. the drainage (-Q) and raising damp (+Q) was obtained from the equation of water balance. The rate of evapotranspiration of pasture was low, yet this was the main system of water output component, followed by runoff, which together totaled approximately 80% of precipitation. This research demonstrated that, even in years of drought it is necessary to quantify the runoff and losses and/or water gains at lower limit of the soil to get accurate assessment of the dynamics of water in soil-pasture system in semi-arid conditions. Palavras-chave:escoamento superficial evapotranspiração drenagem armazenamento de água no solo

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