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.
The São Francisco River is strategically important due to its hydroelectric potential and for bringing the largest water body of Brazilian Semiarid region, supplying water for irrigation, urban, and ...industrial activities. Thereby, for the purpose of characterizing changes on the precipitation patterns over São Francisco River basin, 11 extremes precipitation indices as defined by the joint WMO/CCI/ETCCDMI/CLIVAR project were calculated using daily observation from the 59 rain gauges during 1947–2012 period. The extreme climatic indices were calculated with the RClimDex software, which performs an exhaustive data quality control, intending to identify spurious errors and dataset inconsistencies. Weak and significant regional changes were observed in both CDD and SDII indices. Most precipitation extremes indices decreased but without statistical significance. The spatial analysis of indices did not show clearly regional changes due to the complexity of hydrometeorology of the region. In some cases, two rainfall stations exhibited opposite trends with the same significance level although they are separated by a few kilometers. This has occurred more frequently in Lower-Middle São Francisco, probably associated with intense land cover change over the last decades in this region.
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).
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.
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.
► A field study on irrigated cotton was carried out to obtain evapotranspiration and crop coefficient values. ► The accumulated crop evapotranspiration was 716 and 754mm for 2008 and 2009 years, ...respectively. ► The crop coefficient values were 0.75, 1.09 and 0.80 for initial, middle and end, of growth season, respectively. ► Crop evapotranspiration values obtained from FAO crop coefficient adjusted to local conditions were overestimated. ► The irrigation scheduling based on the FAO crop coefficient would require about 10h of additional irrigation.
During the twentieth century, the cotton crop was the main agricultural product in the semiarid regions of Brazil, with over 3.2 million hectares planted. However, due to structural problems, this activity became uncompetitive and economically unfeasible, being virtually wiped out in the eighties. The revival of cotton growing in semiarid lands of Brazil is important to the regional economy. However, the adoptions of new technologies mainly related to the water use efficiency are needed. Thus, accurate ETc estimates are required for efficient irrigation management. The Kc method is a practical and reliable technique for estimating ETc, and has been vastly applied by the farmers in the semiarid lands of Brazil. However, the use of Kc values listed in FAO-56 can contribute to ETc estimates that are substantially different from actual ETc. Hence the importance of determining Kc values experimentally. A field study on sprinkler-irrigated cotton was carried out during the dry seasons of 2008 and 2009 years in the Apodi Plateau, Brazilian semiarid lands. This study aims to determine ETc and the Kc curve values using the Bowen Ratio Energy Balance (BREB) technique. The locally developed Kc curves are compared with generalized FAO Kc values adjusted for local climate and management. The ETc values were 716mm and 754mm in 2008 and 2009, respectively. These values were higher than those observed in other areas of Brazilian semiarid. These differences are attributed to weather heterogeneity in the region. The average of Kc values were 0.75, 1.09 and 0.80 for initial, middle and end, of growing season, respectively. These values were lower than the Kc-FAO-Adjusted to local conditions. For this reason, ETc values obtained from Kc-FAO-Adjusted were overestimated by 12% in both the years. The irrigation scheduling based on the Kc-FAO-Adjusted increases production cost and yield loss.
Studying the dynamics of gross primary production (GPP) in seasonally dry tropical forests is of fundamental importance to understand the carbon dioxide (CO
2
) balance in this ecosystem, helping ...mitigate its potential impacts at the regional and global levels. Thus, the objective of this work was to evaluate the accuracy of GPP estimated via remote sensing in the Caatinga biome. A set of observed data retrieved from micrometeorological towers equipped with eddy covariance systems were used to validate remote sensing data. The set was measured in a preserved Caatinga fragment. Remotely sensed GPP data was retrieved from the MOD17A2 version 6.0 product of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard the Terra satellite. The validation of MOD17A2 GPP estimates was carried out through the comparison with micrometeorological data measured in situ. In the Caatinga site the comparison between the two GPP data types showed a moderate correlation with Pearson's correlation coefficient (r) = 0.65 and coefficient of determination (R
2
) = 0.43 and the product performed better in representing GPP in the Caatinga during the dry season. Results showed that although the MOD17A2 product represents the annual behaviour of GPP, the algorithm could be improved in order to provide GPP information that is more similar to surface measured data over these land covers.
Arid and semi-arid environments correspond to one-third of the Earth’s terrestrial surface. In these environments, precipitation is an essential and limiting element for vegetation growth and ...ecosystem biomass productivity. The semi-arid region of Brazil comprises around 11.5% of the national territory, where the Caatinga biome originally composed ~ 76% of this area, with water deficit as a prominent feature, annual rainfall lower than 800 mm, temperatures ranging between 25 and 30 °C, and potential evapotranspiration higher than 2000 mm/year. Research on the dynamics of mass and heat fluxes through techniques such as eddy covariance (EC) has contributed to estimate the magnitude and seasonal patterns of turbulent exchanges between ecosystems and the atmosphere. This study was conducted in an area of dense Caatinga (DC) and another of sparse Caatinga (SC) from 2013 to 2014. It was observed that albedo (
α
) and net radiation (
R
n
) were higher in the SC compared with DC since the magnitude of incoming shortwave radiation was higher in this area. It was found that most of the
R
n
is converted to sensible heat flux (
H
), mainly during the dry period in the SC, about 50% for
H
and 20% for λ
E
. The energy balance closure showed that the turbulent fluxes (
H
+ λ
E
) were underestimated in comparison to the available energy at the surface (
R
n
−
G
). We also observed that this discrepancy was higher in the DC area, corresponding to ~ 30%.
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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