Trends and extremes of drought episodes in Vietnam sub-regions during 1980–2017 at different timescales

This study investigated the temporal occurrence of dry conditions in the seven climatic sub-regions of Vietnam during the 1980–2017 period. This assessment was performed using the Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) at 1 to 24 months timescales. Results show that the main periods of extreme drought occurred simultaneously throughout the country in 1992–1993 and 2003–2004, except for 2015–2016, when it was not identified in the southern region. In addition, a slight temporal lag was identified latitudinally (north–south) at the beginning of dry conditions, revealing the largest difference between the northern and southern regions. A positive trend in the time series of both indices (SPEI and SPI) prevailed in all sub-regions, with the SPEI minus SPI difference always being negative, suggesting the importance of temperature and evapotranspiration for this trend. Further detailed analyses were then performed using SPEI at 1-month and 12-months timescales for all climate sub-regions, as well as the main indicators to characterize duration and severity. Results show that the number of drought episodes did not vary much between regions, but they did vary in duration and severity at the annual scale. Moreover, changes in the soil root zone are largely associated with dry and wet conditions not only from season to season, but also in longer accumulation periods and more strongly in the northern regions of Vietnam. Indeed, a study of the most severe drought episodes also revealed the occurrence of negative anomalies of the root-soil moisture in the subsequent four or more months. Dynamic atmospheric conditions associated with the peak of most severe drought episodes show the crucial role of subsidence of dry air in the middle and high atmosphere, which prevents convection in the lower troposphere. Finally, the linkages between drought conditions in Vietnam and large-scale atmospheric and oceanic teleconnection patterns were revealed to be quite different among northern and southern sub-regions. During the positive phase of El Niño–Southern Oscillation (ENSO), drought episodes at different timescales were identified in the southern climate sub-regions, while the negative phase was associated with drought conditions in the northern regions.Xunta de Galicia | Ref. ED481B 2019/070Fundação para a Ciência e a Tecnología | Ref. UIDB/50019/2020–IDLXunta de Galicia | Ref. ED481B 2018/062Xunta de Galicia | Ref. ED431C 2017/64-GRCMinisterio de Ciencia, Innovación y Universidades | Ref. RTI2018-095772-B-I00Fundação para a Ciência e a Tecnología | Ref. PTDC/CTA-MET/29233/201

Evaluation of meteorological drought and flood scenarios over Kenya, East Africa

This work examines drought and flood events over Kenya from 1981 to 2016 using the Standardized Precipitation–Evapotranspiration Index (SPEI). The spatiotemporal analysis of dry and wet events was conducted for 3 and 12 months. Extreme drought incidences were observed in the years 1987, 2000, 2006, and 2009 for SPEI-3, whilst the SPEI-12 demonstrated the manifestation of drought during the years 2000 and 2006. The SPEI showed that the wettest periods, 1997 and 1998, coincided with the El Nino event for both time steps. SPEI-3 showed a reduction in moderate drought events, while severe and extreme cases were on the increase tendencies towards the end of the twentieth century. Conversely, SPEI-12 depicted an overall increase in severe drought occurrence over the study location with ab observed intensity of −1.54 and a cumulative frequency of 64 months during the study period. Wet events showed an upward trend in the western and central highlands, while the rest of the regions showed an increase in dry events during the study period. Moreover, moderate dry/wet events predominated, whilst extreme events occurred least frequently across all grid cells. It is apparent that the study area experienced mild extreme dry events in both categories, although moderately severe dry events dominated most parts of the study area. A high intensity and frequency of drought was noted in SPEI-3, while the least occurrences of extreme events were recorded in SPEI-12. Though drought event prevailed across the study area, there was evidence of extreme flood conditions over the recent decades. These findings form a good basis for next step of research that will look at the projection of droughts over the study area based on regional climate models

Global Warming and 21st Century Drying

Global warming is expected to increase the frequency and intensity of droughts in the twenty-first century, but the relative contributions from changes in moisture supply (precipitation) versus evaporative demand (potential evapotranspiration; PET) have not been comprehensively assessed. Using output from a suite of general circulation model (GCM) simulations from phase 5 of the Coupled Model Intercomparison Project, projected twentyfirst century drying and wetting trends are investigated using two offline indices of surface moisture balance: the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI). PDSI and SPEI projections using precipitation and Penman- Monteith based PET changes from the GCMs generally agree, showing robust cross-model drying in western North America, Central America, the Mediterranean, southern Africa, and the Amazon and robust wetting occurring in the Northern Hemisphere high latitudes and east Africa (PDSI only). The SPEI is more sensitive to PET changes than the PDSI, especially in arid regions such as the Sahara and Middle East. Regional drying and wetting patterns largely mirror the spatially heterogeneous response of precipitation in the models, although drying in the PDSI and SPEI calculations extends beyond the regions of reduced precipitation. This expansion of drying areas is attributed to globally widespread increases in PET, caused by increases in surface net radiation and the vapor pressure deficit. Increased PET not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone. This PET amplification effect is largest in the Northern Hemisphere mid-latitudes, and is especially pronounced in western North America, Europe, and southeast China. Compared to PDSI projections using precipitation changes only, the projections incorporating both precipitation and PET changes increase the percentage of global land area projected to experience at least moderate drying (PDSI standard deviation of or = -1; 11 to 44 %), although this is likely less meaningful because much of the PET induced drying in the SPEI occurs in the aforementioned arid regions. Integrated accounting of both the supply and demand sides of the surface moisture balance is therefore critical for characterizing the full range of projected drought risks tied to increasing greenhouse gases and associated warming of the climate system

Tools and metrics to characterize extreme climate events and evaluate climatic datasets over the Upper Colorado River Basin for societal applications

This study supports the Denver Water management goals by providing tools and metrics that are relevant for operational activities. The study focuses mainly on drought events, but with a slight mention of pluvial conditions over upper Colorado River basin (UCRB), the region that supplies water to Denver community. The study uses observed monthly minimum and maximum temperatures and monthly precipitation datasets (Climatic Research Unit; CRU and Precipitation-Elevation Regression on Independent Slopes Model; PRISM) and modeling outputs from 34 members of the Community Earth System Model Large Ensemble (CESM-LE) to monitor and characterize droughts over the region. With these datasets, we compute two multi-scalar moisture indices: standardized precipitation evapotranspiration index (SPEI) and standardized precipitation index (SPI) on a 36-month scale. We evaluate the capability of the CESM-LE to reproduce drought over the region using the more recently developed spatial verification tool, the Method for Object-based Diagnostic Evaluation (MODE) technique. In addition, the study examines the large-scale atmospheric circulation features associated with drought and pluvial conditions using reanalysis output. The results reveal the usefulness of these datasets, the drought indicators, and the spatial verification technique as important analytical tools to monitor and characterize extreme hydroclimatic conditions over UCRB

Climatic Controls on Spring Onset of the Tibetan Plateau Grasslands from 1982 to 2008

Understanding environmental controls on vegetation spring onset (SO) in the Tibetan Plateau (TP) is crucial to diagnosing regional ecosystem responses to climate change. We investigated environmental controls on the SO of the TP grasslands using satellite vegetation index (VI) from the 3rd Global Inventory Modeling and Mapping Studies (GIMMS3g) product, with in situ air temperature (Ta) and precipitation (Prcp) measurement records from 1982 to 2008. The SO was determined using a dynamic threshold method based on a 25% threshold of seasonal VI amplitude. We find that SO shows overall close associations with spring Ta, but is also subject to regulation from spring precipitation. In relatively dry but increasingly wetting (0.50 mm·year−1, p \u3c 0.10) grasslands (mean spring Prcp = 22.8 mm; Ta = −3.27 °C), more precipitation tends to advance SO (−0.146 day·mm−1, p = 0.150) before the mid-1990s, but delays SO (0.110 day·mm−1, p = 0.108) over the latter record attributed to lower solar radiation and cooler temperatures associated with Prcp increases in recent years. In contrast, in relatively humid TP grasslands (73.0 mm; −3.51 °C), more precipitation delays SO (0.036 day·mm−1, p = 0.165) despite regional warming (0.045 °C·year−1, p \u3c 0.05); the SO also shows a delaying response to a standardized drought index (mean R = 0.266), indicating a low energy constraint to vegetation onset. Our results highlight the importance of surface moisture status in regulating the phenological response of alpine grasslands to climate warming

Multiyear Droughts and Pluvials over the Upper Colorado River Basin and Associated Circulations

This study analyzes spatial and temporal characteristics of multiyear droughts and pluvials over the southwestern United States with a focus on the upper Colorado River basin. The study uses two multiscalar moisture indices: standardized precipitation evapotranspiration index (SPEI) and standardized precipitation index (SPI) on a 36-month scale (SPEI36 and SPI36, respectively). The indices are calculated from monthly average precipitation and maximum and minimum temperatures from the Parameter-Elevation Regressions on Independent Slopes Model dataset for the period 1950–2012. The study examines the relationship between individual climate variables as well as large-scale atmospheric circulation features found in reanalysis output during drought and pluvial periods. The results indicate that SPEI36 and SPI36 show similar temporal and spatial patterns, but that the inclusion of temperatures in SPEI36 leads to more extreme magnitudes in SPEI36 than in SPI36. Analysis of large-scale atmospheric fields indicates an interplay between different fields that yields extremes over the study region. Widespread drought (pluvial) events are associated with enhanced positive (negative) 500-hPa geopotential height anomaly linked to subsidence (ascent) and negative (positive) moisture convergence and precipitable water anomalies. Considering the broader context of the conditions responsible for the occurrence of prolonged hydrologic anomalies provides water resource managers and other decision-makers with valuable understanding of these events. This perspective also offers evaluation opportunities for climate models

Changes in water and carbon in Australian vegetation in response to climate change

Australia has experienced pronounced climate change since 1950, especially in forested areas where a reducing trend in annual precipitation has occurred. However, the interaction between forests and water at multiple scales, in different geographical locations, under different management regimes and in different forest types with diverse species is not fully understood. Therefore, some interactions between forests and hydrological variables, and in particular whether the changes are mediated by management or climate, remain controversial. This thesis investigates the responses of Australia’s terrestrial ecosystems to both historical and projected climate change using remote sensing data and ecohydrological models. The thesis is structured in seven chapters, and contains five research chapters. Vegetation dynamics and sensitivity to precipitation change on the Australian continent for the past long drought period (2002-2010) are explored in Chapter 2 using multi-resource vegetation indices (VIs; normalized difference vegetation index (NDVI) and leaf area index (LAI)) and gridded climate data. During drought, precipitation and VIs declined across 90% and 80% of the whole continent, respectively, compared to the baseline period of 2000-2001. The most dramatic declines in VIs occurred in open shrublands near the centre of Australia and in southwestern Australia coinciding with significant reductions in precipitation and soil moisture. Overall, a strong relationship between water (precipitation and soil moisture) and VIs was detected in places where the decline in precipitation was severe. For five major vegetation types, cropland showed the highest sensitivity to water change, followed by grassland and woody savanna. Open shrublands showed moderate sensitivity to water change, while evergreen broadleaf forests only showed a slight sensitivity to soil moisture change. Although there was no consistent significant relationship between precipitation and VIs of evergreen broadleaf forests, forests in southeastern Australia, where precipitation had declined since 1997, appear to have become more sensitive to precipitation change than in southwestern Australia. The attribution of impacts from climate change and vegetation on streamflow change at the catchment scale for southwestern Australia are described in Chapter 3. This region is characterized by intensive warming and drying since 1970. Along with these significant climate changes, dramatic declines in streamflow have occurred across the region. Here, 79 catchments were analyzed using the Mann-Kendall trend test, Pettitt’s change point test, and the theoretical framework of the Budyko curve to study changes in the rainfall-runoff relationship, and effects of climate and vegetation change on streamflow. A declining trend and relatively consistent change point (2000) of streamflow were found in most catchments, with over 40 catchments showing significant declines (p < 0.05, -20% to -80%) between the two periods of 1982-2000 and 2001-2011. Most of the catchments have been shifting towards a more water-limited climate condition since 2000. Although streamflow is strongly related to precipitation for the period of 1982 to 2011, change of vegetation (land cover/use change and growth of vegetation) dominated the decrease in streamflow in about two-thirds of catchments. The contributions of precipitation, temperature and vegetation to streamflow change for each catchment varied with different catchment characters and climate conditions. In Chapter 4, the magnitude and trend of water use efficiency (WUE) of forest ecosystems in Australia, and their response to drought from 1982 to 2014, were analyzed using a modified version of the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model in the BIOS2 modelling environment. Instead of solely relying on the ratio of gross primary productivity (GPP) to evapotranspiration (ET) as WUE (GPP/ET), the ratio of net primary productivity (NPP) to Transpiration (ETr) (NPP/ETr) was also adopted to more comprehensively understand the response of vegetation to drought. For the study period, national average annual forest WUE was 1.39 ± 0.80 g C kg−1 H2O for GPP/ET and 1.48 ± 0.28 g C kg−1 H2O for NPP/ETr. The WUE increased in the entire study area during this period (with a rate of 0.003 g C kg−1 H2O yr-1 for GPP/ET; p < 0.005 and a rate of 0.0035 g C kg−1 H2O yr-1 for NPP/ETr; p < 0.01), whereas different trends were detected in different biomes. A significantly increasing trend of annual WUE was only found in woodland areas due to higher magnitudes of increases in GPP and NPP than ET and ETr. The exception was in eucalyptus open forest area where ET and ETr decreased more than reductions in GPP and NPP. The response of WUE to drought was further analyzed using 1-48 month scales standardised precipitation-evapotranspiration index (SPEI). More severe (SPEI < -1) and frequent droughts (over ca. 8 years) occurred in the north than in the southwest and southeast of Australia since 1982. The response of WUE to drought varied significantly regionally and across forest types. The response of WUE to drought varied significantly regionally and across forest types, due to the different responses of carbon sequestration and water consumption to drought. The cumulative lagged effect of drought on monthly WUE derived from NPP/ETr was consistent and relatively short and stable between biomes (< 4 months), but notably varied for WUE based on GPP/ET, with a long time lag (mean of 16 months). As Chapters 2-4 confirmed that climate change has been playing an important role in the water yield and vegetation dynamics in Australia, the response of water yield and carbon sequestration to projected future climate change scenarios were integrated using the Water Supply Stress Index and Carbon model (WaSSI-C) ecohydrology model in Chapter 5. This model was calibrated with the latest water and carbon observations from the OzFlux network. The performance of the WaSSI-C model was assessed with measures of Q from 222 Hydrologic Reference Stations (HRSs) in Australia. Across the 222 HRSs, the WaSSI-C model generally captured the spatial variability of mean annual and monthly Q as evaluated by the Correlation Coefficient (R2 = 0.1-1.0), Nash-Sutcliffe Efficiency (NSE = -0.4-0.97), and normalized Root Mean Squared Error by Q (RMSE/Q = 0.01-2.2). Then 19 Global Climate Models (GCMs) from the Coupled Model Intercomparison Project phase 5 (CMIP5), across all Representative Concentration Pathways (RCPs) (RCP2.6, RCP4.5, RCP6.0 and RCP8.5), were used to investigate the potential impacts of climate change on water and carbon fluxes. Compared with the baseline period of 1995-2015 across the 222 HRSs, the temperature was projected to rise by an average of 0.56 to 2.49 ˚C by 2080, while annual precipitation was projected to vary significantly. All RCPs demonstrated a similar spatial pattern of change of projected Q and GPP by 2080, however, the magnitude varied widely among the 19 GCMs. Overall, future climate change may result in a significant reduction in Q but may be accompanied by an increase in ecosystem productivity. Mean annual Q was projected to decrease by 5 - 211 mm yr-1 (34% - 99%) by 2080, with over 90% of the watersheds declining. On the contrary, GPP was projected to increase by 17 - 255 g C m-2 yr-1 (2% - 17%) by 2080 in comparison with 1995-2015 in southeastern Australia. A significant limitation of WaSSI-C model is that it only runs serially. High resolution simulations at the continental scale are therefore not only computationally expensive but also present a run-time memory burden. In Chapter 6, using distributed (Message Passing Interface, MPI) and shared (Open Multi-Processing, OpenMP) memory parallelism techniques, the model was parallelized (and renamed as dWaSSI-C), and this approach was very effective in reducing the computing run-time and memory use. By using the parallelized model, several experiments were carried out to simulate water and carbon fluxes over the Australian continent to test the sensitivity of the model to input data-sets of different resolutions, as well as the sensitivity of the model to its WUE parameter for different vegetation types. These simulations were completed within minutes using dWaSSI-C, and this would not have been possible with the serial version. Results show that the model is able to simulate the seasonal cycle of GPP reasonably well when compared to observations at 4 eddy flux sites in Australia. The sensitivity analysis showed that simulated GPP was more sensitive to WUE during the Australian summer as compared to winter, and woody savannas and grasslands showed higher sensitivity than evergreen broadleaf forests and shrublands. With the parallelized dWaSSI-C model, it will now be much easier and faster to conduct continental scale analyses of the impacts of climate change and land cover change on water and carbon. Overall, vegetation and water of Australian ecosystems have become very sensitive to climate change after a considerable decline in streamflow. Australian ecosystems, especially in temperate Australia, are projected to experience warmer and drier climate conditions with increasing drought risk. However, the prediction from different models varied significantly due to the uncertainty of each climate model. The impacts of different forest management scenarios should be studied to find the best land use pattern under the changing climate. Forest management methods, such as thinning and reforestation, may be conducted to mitigate the impacts of drought on water yield and carbon sequestration in the future

Regionalization and Spatiotemporal Variation of Drought in China Based on Standardized Precipitation Evapotranspiration Index (1961–2013)

China is considered to be one of the most drought prone countries. This study is dedicated to analyzing the regionalization and spatiotemporal variations of drought based on the Standardized Precipitation Evapotranspiration Index, which covers the period 1961–2013 across 810 stations in China. Using Spatial “K”luster Analysis by Tree Edge Removal method, China was divided into eight regions: southwest (SW), northeast (NE), north (N), southeast (SE), Yangtze River (YR), northwest (NW), central China (C), and Tibet Plateau (TP). The spatiotemporal variations of drought characteristics indicated that the drought count in NE and C was generally high. Southern China and NW had suffered long drought duration and extreme severity. The MK test results show that stations with significant drying trends mainly locate in SW, N, NW, and C. The severe drought frequency was very high in 1990s and 2000s. Furthermore, more attention should be paid to abnormal less precipitation in summer and abnormal high temperature in spring in SW, NE, N, and C. Besides, abnormal less precipitation is the main factor of drought in SE and YR in whole year. This study is anticipated to support the water resources management, and to promote the realization of environmental protection and agricultural production

Potential impacts of climate change and land-use change on hydrological drought in the Western Cape (South Africa)

The Western Cape (South Africa) recently witnessed the most severe drought on record. The meteorological drought, which was characterised by below-normal rainfall for three consecutive years (2015 – 2017), cascaded to agricultural and then hydrological drought, resulting in devastating socio-economic consequences. While some studies indicate that climate change may increase the severity and frequency of droughts in the Western Cape in the future, there is a lack of information on how to mitigate the effects of future climate change on hydrological drought. This dissertation therefore investigated the extent to which land-use changes could be applied to reduce climate change impacts on future hydrological drought in this region. For the study, the revised Soil Water Assessment Tool (SWAT+) was calibrated and evaluated over four river basins in the Western Cape, and the climate simulation dataset from the COordinated Regional Downscaling EXperiment (CORDEX) was bias-corrected. Using the bias-corrected climate data as a forcing, the SWAT+ was used to project the impacts of future climate change on water yield and hydrological drought in the four basins and to quantify the sensitivity of the projection to four feasible land-use change scenarios in these basins. The relevant land-use scenarios are the expansion of mixed forests (FrLand), the restoration of grassland (GrLand), the restoration of shrubland (SrLand), and the expansion of cropland (CrLand). The model evaluation shows good agreement between the simulated and observed monthly streamflow at hydrological stations, and the bias correction of the CORDEX datasets improved the quality of the SWAT+ hydrological simulations in the four basins. The climate change projection depicts an increase in temperature and potential evapotranspiration but a decrease in precipitation and all the hydrological variables. Drying is projected across the Western Cape, and the magnitude of such drying increases with higher global warming levels (GWLs). The land-use changes alter the impacts of climate change by influencing the hydrological balance. While FrLand mitigates the impacts of climate change on the frequency of hydrological drought by increasing streamflow, soil water and percolation, CrLand mitigates the impacts by increasing surface runoff. However, the magnitudes of these land-use change impacts are very small compared to the climate change impacts. Hence, the results suggest that land-use changes may not be an efficient strategy for mitigating the climate change impacts on hydrological drought over the region. The findings obtained from this 2 research provide relevant information towards mitigating the severity of future droughts and improving water security in Western Cape River Basins