Land degradation in semi-arid natural environments is usually associated with climate vulnerability and anthropic pressure, leading to devastating social, economic and environmental impacts. In this ...sense, remotely sensed vegetation parameters, such as the Normalized Difference Vegetation Index (NDVI), are widely used in the monitoring and forecasting of vegetation patterns in regions at risk of desertification. Therefore, the objective of this study was to model NDVI time series at six desertification hotspots in the Brazilian semi-arid region and to verify the applicability of such models in forecasting vegetation dynamics. We used NDVI data obtained from the MOD13A2 product of the Moderate Resolution Imaging Spectroradiometer sensor, comprising 16-day composites time series of mean NDVI and NDVI variance for each hotspot during the 2000-2018 period. We also used rainfall measured by weather stations as an explanatory variable in some of the tested models. Firstly, we compared Holt-Winters with Box-Jenkins and Box-Jenkins-Tiao (BJT) models. In all hotspots the Box-Jenkins and BJT models performed slightly better than Holt-Winters models. Overall, model performance did not improve with the inclusion of rainfall as an exogenous explanatory variable. Mean NDVI series were modelled with a correlation of up to 0.94 and a minimum mean absolute percentage error of 5.1%. NDVI variance models performed slightly worse, with a correlation of up to 0.82 and a minimum mean absolute percentage error of 22.0%. After the selection of the best models, we combined mean NDVI and NDVI variance models in order to forecast mean-variance plots that represent vegetation state dynamics. The combined models performed better in representing dry and degraded vegetation states if compared to robust and heterogeneous vegetation during wet periods. The forecasts for one seasonal period ahead were satisfactory, indicating that such models could be used as tools for the monitoring of short-term vegetation states.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
Understanding the long-term behavior of rainfall and potential evapotranspiration (PET) over watersheds is crucial for the monitoring of hydrometeorological processes and climate change at the ...regional scale. The São Francisco watershed (SFW) in Brazil is an important hydrological system that transports water from humid regions throughout the Brazilian semiarid region. However, long-term, gapless meteorological data with good spatial coverage in the region are not available. Thus, gridded datasets, such as the Climate Research Unit TimeSeries (CRU TS), can be used as alternative sources of information, if carefully validated beforehand. The objective of this study was to assess CRU TS (v4.02) rainfall and PET data over the SFW, and to evaluate their long-term (1942–2016) climatological aspects. Point-based measurements retrieved from rain gauges and meteorological stations of national agencies were used for validation. Overall, rainfall and PET gridded data correlated well with point-based observations (r = 0.87 and r = 0.89), with a poorer performance in the lower (semiarid) portion of the SFW (r ranging from 0.50 to 0.70 in individual stations). Increasing PET trends throughout the entire SFW and decreasing rainfall trends in areas surrounding the semiarid SFW were detected in both gridded (smoother slopes) and observational (steeper slopes) datasets. This study provides users with prior information on the accuracy of long-term CRU TS rainfall and PET estimates over the SFW.
Partially located in the Brazilian Semiarid region, the São Francisco watershed (SFW) is a strategic hydrological system for power generation, agricultural development and the integration of the ...national territory. Droughts are recurrent in the Brazilian Semiarid, and climate change projected scenarios indicate their intensification in the SFW, which may compromise the access to its resources. In this context, the objective of this study was to characterize long‐term spatial and temporal meteorological drought patterns over the SFW while accounting for its different climates. We used gridded Climate Research Unit Time Series rainfall and potential evapotranspiration (PET) data for the 1942–2019 (78 years) period to retrieve a monthly climatic water balance‐based drought index. The climatological aspects of drought incidence in the SFW were assessed through linear trend analysis, the estimation of the relative effects of precipitation and PET trends in drought occurrence, and the identification of the main interannual variability modes controlling the triggering of meteorological drought through wavelet functions. Results showed that water deficit periods are becoming more frequent and intense, affecting larger areas in the middle (tropical climate) and lower (semiarid climate) portions of the SFW. Months with water surplus in these regions are also becoming less frequent. In the upper (humid subtropical climate) SFW, evidences show that water deficit periods are becoming more frequent, with an expansion of areas affected by droughts. In this subregion, increasing PET trends played a more important role than precipitation trends in the propagation of drought. Finally, low‐frequency teleconnection mechanisms such as the El Niño–Southern Oscillation were found to drive the occurrence of drought in most of the SFW, while higher frequency oscillations (such as the Atlantic Meridional Mode) play a role in the lower latitudes of the basin. In the upper SFW, results indicated that interannual variability might be driven by oscillations with stronger intraseasonal components.
Climate types in the São Francisco watershed and mean climatological behaviour for each subregion: upper São Francisco (USF); middle São Francisco (MSF); lower‐middle São Francisco (LMSF); and lower São Francisco (LSF). Adapted from Alvares et al. (2014) and Maneta et al. (2009) using CRU TS v4.05 data from 1942 to 2019 (data grid is shown in the background).
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The identification of spatial and temporal rainfall climatology patterns is crucial for hydrometeorological studies over semiarid watersheds, which frequently face water distribution conflicts and ...socioeconomic issues due to water scarcity. Thus, the objective of this study was to propose a comprehensive approach for the characterization of rainfall climatology over semiarid watersheds. Monthly rainfall time series (1962–2015) with up to 30% of gaps measured in 56 rain gauges in the Piranhas-Açu Watershed—Brazilian semiarid region—were used. Data gaps were filled through a combination of simple spatial interpolation techniques. Principal component analysis and cluster analysis identified two homogeneous rainfall subregions in the basin: C1, in the upper portion, and C2, in the middle and lower portions. Rainfall volumes in C2 were up to 23.5% smaller than those in C1, due to orographic structures which contribute to aridity in this region. Rainfall anomalies were calculated in each cluster through the modified Rainfall Anomaly Index (mRAI) and were associated with the phases of the El Niño–Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM). In years when the ENSO (AMM) was in its positive (negative) phase, there was a higher probability of occurrence of months with above-average rainfall, while the opposite was also true. Results showed that the effects of the patterns are mutually influenced, which has been previously found at larger scales. Finally, mRAI trends were identified through the Mann-Kendall test, which indicated significant negative trends in C1 and C2, especially during the wet season.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Abstract
Forest ecosystems sequester large amounts of atmospheric CO
2
, and the contribution from seasonally dry tropical forests is not negligible. Thus, the objective of this study was to quantify ...and evaluate the seasonal and annual patterns of CO
2
exchanges in the
Caatinga
biome, as well as to evaluate the ecosystem condition as carbon sink or source during years. In addition, we analyzed the climatic factors that control the seasonal variability of gross primary production (GPP), ecosystem respiration (R
eco
) and net ecosystem CO
2
exchange (NEE). Results showed that the dynamics of the components of the CO
2
fluxes varied depending on the magnitude and distribution of rainfall and, as a consequence, on the variability of the vegetation state. Annual cumulative NEE was significantly higher (p < 0.01) in 2014 (−169.0 g C m
−2
) when compared to 2015 (−145.0 g C m
−2
) and annual NEP/GPP ratio was 0.41 in 2014 and 0.43 in 2015. Global radiation, air and soil temperature were the main factors associated with the diurnal variability of carbon fluxes. Even during the dry season, the NEE was at equilibrium and the
Caatinga
acted as an atmospheric carbon sink during the years 2014 and 2015.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Plant species of the Brazilian Caatinga experience seasonal wet and dry extremes, requiring seasonally different leaf characteristics for optimizing water availability. We investigated if Croton ...blanchetianus Baill exhibits leaf morphoanatomical traits across seasons and positioning in sunlight/natural shade. Leaves of ten 1-3 m tall plants in full sunlight and ten in natural shade were assessed in May, July (wet season), October and December (dry season) 2015 for gas exchange, leaf size, lamina and midrib cross sections (14 parameters), and chloroplast structure (5 parameters). Net photosynthesis was greater during the wet season (21.6 µm
s
) compared to the dry season (5.8 µm
s
) and was strongly correlated with almost all measured parameters (p < 0.01). Shaded leaves in the wet season had higher specific leaf area (19.9 m
kg
in full-sun and 23.1 m
kg
in shade), but in the dry season they did not differ from those in full sun (7.5 m
kg
and 7.2 m
kg
). In the wet season, the expansion of the adaxial epidermis and mesophyll lead to larger and thicker photosynthetic area of leaves. Furthermore, chloroplast thickness, length and area were also significantly larger in full sunlight (2.1 μm, 5.1 μm, 15.2 μm
; respectively) and shaded plants (2.0 μm, 5.2 μm, 14.8 μm
; respectively) during wetter months. Croton blanchetianus exhibits seasonal plasticity in leaf structure, presumably to optimize water use efficiency during seasons of water abundance and deficit. These results suggest that the species is adaptable to the increased drought stress projected by climate change scenarios.
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The lowest water availability area in Brazil is the Northeast Atlantic Eastern Hydrographic Region (NAERH). It plays a fundamental role in the lives of 24.1 million inhabitants spread throughout 874 ...cities. Drought is recurrent in this semiarid climate, affecting agriculture, biodiversity, the ecosystem and other environmental spheres. Therefore, the goal of this research is to combine different drought indexes to quantify drought intensity and duration in the NAERH. Besides the traditionally used rainfall data, multi-temporal data from the Gravity Recovery and Climate Experiment (GRACE) and Global Positioning System (GPS) were also used. The indexes are the Combined Climatic Deviation Index (CCDI), Drought Severity Index (DSI) and Vertical Crustal Deformation Index (DIVCD). The Standardized Precipitation Index (SPI) was used for validation of the other indexes through the Spearman rank correlation, which retrieved ρ = 0.76 and 0.68 between the CCDI and the SPI-03/06. On the other hand, DSI correlated with the SPI-24/36 with ρ = 0.67/0.75. Despite limitations, the DIVCD accurately detected the frequencies of hydrological droughts. All indexes identified the last severe drought from 2012 to 2018, and its persistence throughout 2019 and 2020. The combined indexes approach reveals nuances of the indexes, improving the baseline to thoroughly understand drought at different temporal scales.
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•The CCDI correlated with short time scales, such as SPI-03/06.•The DSI is suitable for longer time scales i.e. SPI-24/36.•GPS stations can potentially monitor hydrological drought.•The multi-indexes were able to detect the 2012–2018 drought.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Research focusing on energy, water vapor, and CO2 exchanges over the Caatinga Biome is recent. Consequently, there exist research gaps on this topic that limit our ability to understand and project ...interannual and long-term variations in the CO2 cycle of this environment. Thus, the objective of this study was to evaluate the behavior of the energy and CO2 balances of the Caatinga Biome under extreme rainfall conditions (extreme drought and intense rainfall) using simulations of the SITE model (Simple Tropical Ecosystem Model). Meteorological data from the years 2009 (heavy rainfall) and 2012 (extreme drought) obtained from an INMET automatic station were used. Relationships between monthly GPP and NEE values and rainfall were also analyzed. Although the SITE model had been previously calibrated for the region, adjustments to the calibration were necessary for our study due to the extreme climatic values of the selected years. The results highlight the impact of rainfall on energy and mass exchanges over the Caatinga, particularly evident in the partitioning of the energy balance and the CO2 balance. In 2009 (during heavy rainfall), the fraction of Rn converted into LE was nearly 6% higher compared to the values observed in 2012 (during extreme drought). Regarding CO2, it was found that the Caatinga behaved as a sink, even under extreme drought conditions (2012), with annual average values of −1.86 µmol m−2 s−1 (2009) and −0.81 µmol m−2 s−1 (2012). Relationships between monthly GPP and NEE values and rainfall were also investigated, revealing an asymptotic relationship between the components of CO2 balance and rainfall. It was evident that both the monthly values of GPP and NEE tend to stabilize when monthly rainfall volumes exceed 200 mm.
Although seasonally dry tropical forests are considered invaluable to a greater understanding of global carbon fluxes, they remain as one of the ecosystems with the fewest observations. In this ...context, ecological and ecosystem models can be used as alternative methods to answer questions related to the interactions between the biosphere and the atmosphere in dry forests. The objective of this study was to calibrate the simple tropical ecosystem model (SITE) and evaluate its performance in characterizing the annual and seasonal behavior of the energy and carbon fluxes in a preserved fragment of the Caatinga biome. The SITE model exhibited reasonable applicability to simulate variations in CO2 and energy fluxes (r > 0.7). Results showed that the calibrated set of vegetation parameters adequately simulated gross primary productivity (GPP) and net ecosystem CO2 exchange (NEE). The SITE model was also able to accurately retrieve the time at which daily GPP and NEE peaked. The model was able to simulate the partition of the available energy into sensible and latent heat fluxes and soil heat flux when the calibrated parameters were used. Therefore, changes in the dynamics of dry forests should be taken into consideration in the modeling of ecosystem carbon balances.
•Eddy covariance energy balance closure was investigated in a seasonally dry tropical forest.•Energy balance closure was better during the wet season.•The best energy balance closure occurred under ...very unstable conditions.•Under stable conditions the closure was poor, although the amount of information analyzed under this condition was reduced.•The largest portion of net radiation was converted into sensible heat flux, except during the wet season.
The energy balance closure obtained through the eddy covariance method is a problem which persists, despite advances in the development and improvement of instruments and recent efforts in the description of corrections and in the characterization of measuring uncertainties. In most places the sum of sensible and latent heat fluxes (H and λE) is less than available energy, i.e. the difference between net radiation (Rn) and soil heat flux (G). This study analyzed the annual and seasonal behavior of the energy partitioning and energy balance closure in the Caatinga Biome, which is a seasonally dry tropical forest located in the semiarid lands of Brazil, using the eddy covariance method. Results showed high seasonal variability in the energy partitioning. During the dry season, approximately 70% of Rn was converted into H and less than 5% of it was converted into latent heat flux (λE). During the wet season, the Rn portion converted into H and λE was similar: ˜ 40%. In annual terms, the Rn portion converted into H and λE was of the order of 50% and 20% respectively. The degree of the energy balance closure varied depending on the method used. When the closure was calculated using orthogonal regressions, the slope varied from 0.87 to 0.90 in 2014 and from 0.92 to 1.00 in 2015. However, when the closure was calculated by the energy balance ratio method, values varied from 0.70 to 0.79 in 2014 and from 0.73 to 0.82 in 2015. The closure was better in 2015 if compared to 2014 possibly due to the more intense turbulence observed in 2015 because friction velocity was higher than in 2014. The better closure in 2015 may also be associated with large eddies, which were more frequent in 2014 as evidenced by the correction coefficients for vertical wind velocity and water vapor and vertical wind velocity and sonic temperature. The energy balance closure was also analyzed considering atmospheric instability conditions and the best results were found under very unstable conditions, while the least expressive results were found under stable conditions. Under these conditions negative values of the energy balance ratio were also observed during dry and transition seasons, indicating that fluxes were reversed during these periods.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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