Regional Drought Research Articles

A dryness index TSWDI based on land surface temperature, sun-induced chlorophyll fluorescence, and water balance

Drought has a serious impact on the health of terrestrial ecosystems, socio-economy and human health. It is important to accurately monitor drought conditions. Soil moisture (SM) can directly characterize surface dryness and wetness. Precipitation (PPT), evapotranspiration (ET), land surface temperature (LST), and vegetation status indirectly relate to SM, and they influence each other. Therefore, using one index cannot comprehensively and objectively reflect the real situation of surface dryness and wetness. In this study, based on the Z-score data of LST, OCO-2-based solar-induced chlorophyll fluorescence product (GOSIF) and water balance (WB) from 2003 to 2018, a new three-dimensional (3D) drought index, namely temperature-SIF-Water Balance Dryness Index (TSWDI), was constructed using Euclidean distance method. The accuracy of TSWDI was verified by soil moisture (SM), standardized precipitation index (SPI), and standardized precipitation evapotranspiration index (SPEI), and compared with other recognized drought indices. The results showed that TSWDI had a higher correlation with SM (R = 0.340) and SPEI (R = 0.533) than vegetation health index (VHI) (R was 0.201 and 0.207, respectively), temperature-vegetation-soil moisture dryness index (TVMDI) (R was −0.022 and −0.031, respectively), and temperature vegetation precipitation dryness index (TVPDI) (R was 0.123 and 0.132, respectively). Furthermore, taking the frequency of drought occurrences monitored by SPEI and the normalized anomaly of SM as the benchmark, the errors of drought frequency between TSWDI/TVMDI/TVPDI/VHI and SM/SPEI were obtained, respectively. The results showed that the mean error between TSWDI and SM, SPEI were 0.12 and 0.21, respectively, which were lower than that of VHI (0.19, 0.24), TVMDI (0.87, 0.63) and TVPDI (0.89, 0.69). These all indicated that TSWDI could better monitor regional drought conditions than VHI, TVMDI, and TVPDI in our study area. TSWDI had good applicability in monitoring meteorological and agricultural drought. In addition, the spatial variations and patterns of drought conditions in the study area were explored by the annual time series of TSWDI. The results showed that the study area became wetter at an average rate of 0.0064/year. The pixels reaching significant wetting (P < 0.05) accounted for 49.18%, and only 12.14% of the area reached significant drying (P < 0.05), which was consistent with that of SPEI. Finally, the results from the cross-correlation method indicated that the percentage of TSWDI which was ahead, synchronized, and lag of SM changes were 10.13%, 89.47%, and 0.4%, respectively. TSWDI was further employed to identify drought events in China. It demonstrated that TSWDI and VHI can more accurately capture the onset, evolution, and end of drought than TVMDI and TVPDI in the five southwestern provinces of China. Moreover, R (0.627) between TSWDI and the normalized anomaly of SM was higher than that of VHI (R = 0.619). It indicated that TSWDI can recognize more accurately than TVMDI, TVPDI, and VHI for drought events in the five southwestern provinces of China. Therefore, TSWDI can more accurately monitor regional drought conditions and respond in advance or synchronously to SM changes. TSWDI contains several key indicators (SIF, LST, and WB) of surface drought, in which SIF can reflect the physiological characteristics of vegetation, temperature can affect vegetation transpiration and WB affects vegetation absorbing water from the soil. When drought occurs, TSWDI can synthesize the changes in these three indicators. It can be concluded that TSWDI has great potential for evaluating drought conditions in China.

Human‐Driven Fire Regime Change in the Seasonal Tropical Forests of Central Vietnam

AbstractTo better understand fire regimes and their relation to climate in the seasonal tropical forests of continental Southeast Asia, we developed the first multi‐century tree‐ring based fire history chronology for the region. The chronology included 776 fire scars collected at Bidoup NuiBa National Park (BNNP) in the Central Highlands of Vietnam and spans the period 1636–2020. Fires were recorded in 116 years, representing 47% of the years covered by the 249‐year period between the first fire scar (1772) and the last (2020). While only 9% of years within the sampled BNNP forests experienced fires before 1905, 70% recorded fires between 1906 and 1963 and 90% showed evidence of fire after 1963. Fire occurrence was highly correlated with climate indices (wet season Nino 3.4 and dry season regional Palmer Drought Severity Index) during the period 1906–1963, but showed no significant correlation after 1963. Our fire reconstruction from BNNP suggests that the fire regime has shifted from one driven primarily by climate to one in which human activities dominate the occurrence of fire within these seasonal tropical landscapes.

Predicting long term regional drought pattern in Northeast India using advanced statistical technique and wavelet-machine learning approach

Predicting long term regional drought pattern in Northeast India using advanced statistical technique and wavelet-machine learning approach

Drought monitoring based on temperature vegetation dryness index and its relationship with anthropogenic pressure in a subtropical humid watershed in China

Drought has serious impacts on agricultural production and ecological environment in China. Study on the causes of drought and drought monitoring technology is crucial for drought mitigation. This study conducts drought monitoring based on temperature vegetation dryness index (TVDI) and investigates with the impact of anthropogenic pressure in the Poyang Lake basin (PYLB). In the study, the MODIS normalized vegetation index (NDVI) and land surface temperature (LST) products from 2000 to 2021 were selected to construct NDVI-LST feature space processed with elevation correction, from which the monthly TVDI data was calculated. The land use data and nighttime light data were used to construct the anthropogenic pressure index (API). The results indicate that the frequency of drought in the PYLB is higher in the south and lower in the north. According to drought influence scope (DIS), the occurrence frequency of basin-wide drought (DIS ≥ 50%) in the PYLB is the highest, followed by regional drought (33%≤DIS < 50%), partial local drought (25%≤DIS < 33%), and local drought (10%≤DIS < 25%). The basin as a whole showed an obvious wetting trend in spring and a drying trend in autumn. During the study period, API in the north of PYLB was obviously higher than that in the south, and showed a general increasing trend. The contribution of human activities to drought in the PYLB increased exponentially with the improvement of social development level. Meanwhile, the impact of human activities on drought shows spatial differentiation. Ecological protection and green development have been implemented in the northerly region of the PYLB, and anthropogenic pressure inhibited the degree of drought. Oppositely, extensive socioeconomic development in the central and southern parts of the PYLB has caused damage to the environment and indirectly aggravated regional drought. This study makes important contributions to exploring the response between remote sensing drought monitoring and anthropogenic pressure and promoting regional drought mitigation in humid areas.

Germination and seedling growth of Calluna vulgaris is sensitive to regional climate, heathland succession, and drought

AbstractThe coastal heathlands of Northwest Europe are highly valued cultural landscapes, that are critically endangered due to land use and climatic changes, such as increased frequency and severity of drought events. Our study is the first to assess how the germination and early seedling growth of Calluna vulgaris respond to drought. In a factorial design field experiment, we exposed maternal plants to three in‐situ drought treatments (control, 60%, 90% roof coverage), across three successional stages after fire (pioneer, building, mature), and two regions (60°N, 65°N). Seeds from 540 plants within the experiment were, weighed, and exposed to five water potentials, ranging from −0.25 to −1.7 MPa, in a growth chamber experiment. We recorded germination (percentage, rate), seedling growth (above‐ vs. belowground allocation), and seedling functional traits (specific leaf area [SLA], specific root length [SRL]). Overall variation in germination between regions, successional stages, and maternal drought treatments was largely mediated by variation in seed mass. Plants from the northernmost region had higher seed mass and germination percentages. This is indicative of higher investment in seeds, likely linked to the populations' absence of vegetative root sprouting. Seeds from the mature successional stage germinated to lower final percentages than those from earlier successional stages, especially when the maternal plants had been exposed to drought (60% and 90% roof coverage). Exposure to reduced water availability decreased germination percentage and increased the time to 50% germination. Seedlings fully developed in the range −0.25 to −0.7 MPa, with increased root:shoot and lower SRL during reduced water availability, suggesting a resource‐conservative response to drought during the early stages of development. Our results thus suggest a sensitivity to drought during the germination and seedling life‐history stages that may reduce Calluna's ability to re‐establish from seeds as the incidence and severity of droughts are projected to increase under future climates.

Quantifying effects of different types of droughts on gross primary production in China

AbstractThe increasing impact of drought stress on ecosystems with regard to climate change has received considerable attention, which has been demonstrated to have a time‐lag effect on ecosystems. However, the time‐lag effect on different types of droughts and gross primary production (GPP) at large regional scales remains unclear. Thus, using the GOSIF GPP product data of solar‐induced chlorophyll fluorescence, the meteorological drought index (standardized precipitation evapotranspiration index [SPEI]), and the agricultural drought index (soil moisture deficit index [SMDI]) based on the soil moisture data obtained from the Global Land Data Assimilation System, the time‐lag effect of regional drought and GPP and the contribution of drought to GPP variation were analysed in China from 2000 to 2018. Results showed that (1) China's annual GPP increased at an average rate of 9.26 g C m−2 y−1. With regard to the average rate for each climate zone, the largest increase of 30.44 g C m−2 y−1 was observed in the southern subtropical region, and the smallest increase (4.01 g C m−2 y−1) was observed in the Qinghai–Tibetan plateau, with about 53.11% of the regions showing a significant upward trend. In addition, the significantly declining regions were primarily concentrated in urban areas, accounting for about 2.26% of China. (2) SPEI and GPP had an average lag time of 6.28 months, whereas SMDI and GPP had an average lag time of 7.17 months. The comparison demonstrated that SMDI had a greater influence on GPP than SPEI. (3) The average contribution rate of meteorological drought and agricultural drought to the decrease of GPP was 79.37% and 87.06%, and their average contribution rate to the increase of GPP was 39.11% and 50.57%, respectively. This study will provide comprehensive understanding of the effects of drought on carbon and water cycles in terrestrial ecosystems with regard to climate change.

Late Paleocene to early Oligocene fire ecology of the south Mongolian highland

Changes in fire ecology during warm and cold periods in the geological past are important because of their effects on terrestrial ecosystems and the global carbon cycle. We examined the charcoal concentrations of the Erden Obo section in Inner Mongolia to reconstruct the evolution of wildfire and their relationship to the regional vegetation from the Late Paleocene through Early Oligocene. Our data show that fire frequency were relatively high from the end of the Paleocene to the beginning of the Eocene, in accord with other paleofire records worldwide. However, low fire frequency occurred during the Early Eocene Climate Optimum (EECO), coincident with the change in the regional vegetation from shrubland to forest due to the strengthening of the regional rainfall, and we suggest that the humid climate may have been responsible for this decrease. High frequency fire occurred after the Middle Eocene, near-synchronously with the transition of the regional vegetation from forest to steppe. The high-frequency fire was most likely triggered by regional drought during the aridification process after the Middle Eocene. We propose that these temporal changes in the fire ecology were consistent within the northern temperate zone from the Late Paleocene through Early Oligocene, and we suggest that studies of global wildfires need to be evaluated within the context of paleovegetation zones and ecosystem evolution.

Late Quaternary fluvial environments at Abo Arroyo, New Mexico, U.S.A.: response to millennial-scale climate change

ABSTRACT Abo Arroyo is a tributary of the Rio Grande in central New Mexico. Its alluvial sequence is made up of four informal units, their age defined by 44 AMS radiocarbon dates from 12.8 ka to 0.85 ka. The earliest, unit 1, is terminal Pleistocene to early Holocene (12.8 ka to 9.0 ka), including the Scholle wet meadow (12.3 ka to 11.1 ka) related to the Younger Dryas. After an erosional gap in the record from 9 ka to 6 ka, coarse gravel (unit 2) was deposited in the channel (5.8 ka and 4.3 ka) during the middle Holocene period of aridity. Subsequently, the late Holocene wet period is characterized by fine-textured alluvium (unit 3) (3.5 ka to 0.945 ka) with shells of an aquatic snail, indicating persistent stream flow. An upper 2-m zone of dark-colored clayey silt beds with high amounts of organic carbon and carbonate accumulated from 1.7 ka to 0.945 ka. A record of C4 signatures, previously interpreted to indicate dry conditions 1.4 ka to 0.945 ka, is reinterpreted as an interval of exceptionally wet floodplain conditions with C4 grasses and sedges. Abo Arroyo and other studies indicate three major episodes of late-Quaternary channel entrenchment: 1) from the full-glacial to late glacial-Bølling/Allerød (6.1 ky), 2) during the middle Holocene (2.5 ky), and 3) during the late Holocene Medieval Warm Period (0.4 ky), each erosional event less severe and shorter duration than the preceding one, and all three represent a significant change from wet to dry climate. During the Medieval Warm Period (A.D. 900 to 1300), a shallow channel formed in unit 3 alluvium, bracketed by AMS dates A.D. 1005 in unit 3 alluvium and A. D. 1100 in unit 4 channel fill. The channel cutting occurred with the shift from wet to dry climate, although the downcutting event was preceded by 100 years of landscape adjustment to the long drought. The canyon fill was entrenched again, deep and wide, by arroyo cutting in the late nineteenth and early twentieth centuries. The regional 83-year second-century drought (A.D. 100 to A.D. 182), documented by tree rings, shows up in the stable-carbon-isotope record from Abo Arroyo alluvium. The drought was more severe than the 400-year warm period but had little effect on the preserved alluvial record. The less severe but longer-duration Medieval Warm Period resulted in channel cutting at Abo Arroyo and elsewhere in the broad region at that time, but the second-century drought did not. Finally, the similarity of Abo Arroyo and Rio Grande late Holocene alluvial records with parallel stratigraphy, sedimentology, and geochronology illustrates that tributaries and main valleys respond alike and in concert to climate and climate change.

Moisture stress limits radial mixing of non-structural carbohydrates in sapwood of trembling aspen.

Dynamics in non-structural carbohydrate (NSC) pools may underlie observed drought legacies in tree growth. We assessed how aridity influences the dynamics of different-aged NSC pools in tree sapwood at two sites with differing climate conditions ('wet' vs. 'dry') that also experienced widespread regional drought five years earlier. We used an incubation method to measure radiocarbon (Δ14C) in CO2 respired from Populus tremuloides (aspen) tree rings to evaluate NSC storage and mixing patterns, coupled with measurements of NSC (soluble sugars, starch) concentrations and respired δ13C-CO2. At a wet site, CO2 respired from rings formed during 1962-1967 was only ~ 11years old, suggesting deep sapwood mixing of NSCs as starch. At a dry site, total NSC was about one-third of wet site totals, maximum ages in deep rings were lower, and ages more rapidly increased in shallow rings then plateaued. These results suggest historically shallower mixing and/or relatively higher consumption of NSCs under dry conditions. Both sites, however, had similar aged NSC (<1yr) in the most recent six rings, indicative of deep radial mixing following relatively wet conditions during the sampling year. We suggest significant differences in NSC mixing among sites are driven by moisture stress, where aridity reduces NSC reserves and restricts the depth of radial mixing. However, dynamic climate conditions in the southwestern US resulted in more complex radial patterns of sapwood NSC age than previously described. We suggest a novel conceptual framework to understand how moisture variability might influence the dynamics of NSC mixing in the sapwood.

Applicability evaluation of three improved temperature vegetation dryness indices in High Mountain Asia

Study regionHigh Mountain Asia (Qinghai-Tibet Plateau and surrounding areas) (HMA) Study focusSoil dryness and wetness in HMA respond dramatically to global climate change. Monitoring soil moisture can provide a scientific foundation for water security in the plateau, middle, and lower basins. In this study, the applicability of the Temperature Vegetation Dryness Index (TVDI) with three improved protocols was evaluated to rectify the long-standing misuse of the TVDI method for regional drought assessment. New hydrological insights for the regionsThe unsuitability of the TVDI in alpine regions was demonstrated by the nonlinear correlation between soil moisture and the TVDI. Among the three improvement schemes, TVDIm (obtained by controlling the temperature variable) best reflected the temporal and spatial distribution of soil moisture in the growing season of HMA, especially for surface soil moisture in the middle- and high-altitude areas. The TVDI, TVDIf (obtained by controlling the wet edge), and TVDI_A (obtained by controlling the partition) are not suitable for drought evaluation in medium- and long-term regions because the correlations between the three indicators and soil moisture are inconsistent in space and time. This study provides a new method to identify whether the TVDI can effectively evaluate regional drought. It also provides a more comprehensive reference for monitoring soil moisture and evaluating dryness in alpine regions using optical remote sensing.

Regional Drought Analysis with Standardized Precipitation Evapotranspiration Index (SPEI): Gediz Basin, Turkey

In this study, regional drought analysis was performed with the Standard Precipitation Evapotranspiration Index (SPEI) and L moments techniques by using the monthly average temperature and monthly total rainfall amounts collected from five sites in the Gediz basin in Turkey. Using the monthly average temperatures, the potential evapotranspiration (PET) amounts obtained by the Thornthwaite method and the monthly total rainfall amounts are divided into 5 different reference periods as 1, 3, 6, 9 and 12 months. Expressing the difference between rainfall and potential evapotranspiration amounts, the water balance (Di) series shows that almost all of the 9 and 12 months periods suffers from water deficiency and the 3 months period is water excessive. After determining the distributions that provide the best adaptation to the water balance series, according to the SPEI values obtained, near-normal conditions prevailed in all sites, while moderate and severe arid and humid conditions sometimes occur, while extremely humid and arid conditions are rarely seen. In the regional drought analysis using L moment techniques with the SPEI values obtained, a region of 5 sites was accepted and the irregularity and heterogeneity measures showed that the basin was acceptable homogeneous. SPEI values are generally the best fit generalized extreme values (GEV) for 1 and 3 months periods, generalized normal (GNO) for 6 months period, generalized logistics (GLO) for 9 months period, Pearson type 3 (PE3) distributions for 12 months period. According to the regional SPEI values for reference periods, it has been found near-normal in 1.11, 1.25 and 2 years, moderately humid in 1.04 years, very humid conditions for 1.01 and 1.02 years, moderately dry in 4 and 5 years, severe arid in 10 years, and extremely dry conditions in 20 and longer periods.

A global drought-aridity index: The spatiotemporal standardized precipitation evapotranspiration index

Climate change has increased the risk of drought in both arid and non-arid regions of the globe. Therefore, in order to monitor and evaluate global drought and aridity,based on the principle of standardized precipitation evapotranspiration index (SPEI), this paper selected precipitation and potential evapotranspiration data and used a frequency calculation method to develop a multi-scale global drought aridity index (GDAI) that concurrently considers spatial and temporal variation characteristics: the spatiotemporal standardized precipitation evapotranspiration index. The index was compared with the aridity index (AI) and SPEI. The results showed that the correlation coefficients between GDAI and AI were greater than 0.8 in most areas, and the relationship was closer and could reveal the global aridity and wetness zoning. In addition, the correlation between GDAI and SPEI was also better, which could capture the temporal change characteristics of drought. Moreover, this study also compared the grades of GDAI and SPEI with the actual drought grades in Yunnan from September 2009 to August 2010, and found that GDAI was better than SPEI, and GDAI was the most accurately evaluated at the 3 month time scale, with an accuracy rate of 86.11%. Therefore, this index is more helpful for the assessment and monitoring of global drought and aridity.

Quantification of the meteorological and hydrological droughts links over various regions of Iran using gridded datasets.

Drought is a gradual phenomenon that occurs slowly and directly impacts human life and agricultural products. Due to its significant damage, comprehensive studies must be conducted on drought events. This research employs precipitation and temperature from a satellite-based gridded dataset (i.e., NASA-POWER) and runoff from an observation-based gridded dataset (i.e., GRUN) to calculate hydrological and meteorological gical droughts in Iran during 1981-2014 based on the Standardised Precipitation-Evapotranspiration Index (SPEI) and Hydrological Drought Index (SSI) indices, respectively. In addition, the relationship between the meteorological and hydrological droughts is assessed over various regions of Iran. Afterward, this study employed the Long Short-Term Memory (LSTM) method to predict the hydrological drought based on the meteorological drought over the northwest region of Iran. Results show that hydrological droughts are less dependent on precipitation in the northern regions and the coastal strip of the Caspian Sea. These regions also have a poor correlation between meteorological and hydrological droughts. The correlation between hydrological and meteorological drought in this region is 0.44, the lowest value among the studied regions. Also, on the margins of the Persian Gulf and southwestern Iran, meteorological droughts affect hydrological droughts for 4months. Besides, except the central plateau, most regions experienced meteorological and hydrological droughts in the spring. The correlation between droughts in the center of the Iranian plateau, which has a hot climate, is less than 0.2. The correlation between these two droughts in the spring is stronger than in other seasons (CC = 0.6). Also, this season is more prone to drought than other seasons. In general, hydrological droughts occurred one to two months after the meteorological drought in most regions of Iran. LSTM model for northwest Iran showed that the predicted values had a high correlation with the observed values, and their RMSE was less than 1 in this region. CC, RMSE, NSE, and R-square of the LSTM model are 0.7, 0.55, 0.44, and 0.6, respectively. Overall, these results can be used to manage water resources and allocate water downstream to deal with hydrological droughts.

Recovery of ecosystem carbon and water fluxes after drought in China

Drought recovery time is a crucial indicator of ecosystem resilience. However, existing drought recovery quantification methods have some limitations in terms of drought identification and selection of recovery factors. In addition, drought recovery time in China and related impacts on different landcover types are largely unknown. Therefore, the present study sought to develop a new drought recovery quantification system that considers recovery of ecosystem carbon- and water-related variables in China across seasons (wet and dry) and landcover types. The results showed that drought conditions worsened, especially in northern and western China, from 1982 to 2016. Drought had a strong effect on soil water storage (SWS), with runoff and SWS decreasing in more than 65% of the study area after drought. Drought led to decreases in gross primary productivity (GPP) and water use efficiency (WUE) over 58.7% and 45.2% of the total area, respectively. The mean drought recovery time of SWS (10.4 months) was much longer than that of GPP (3.2 months), WUE (3.7 months), and runoff (2.5 months), especially in northern China. Drought-reduced carbon and water needed more time to recover from drought in grasslands than in other landcover types due to the lower water availability and high evaporative demand. The mean recovery time after dry season drought was longer than that after wet season drought. In a comparison of recent (2001–2016) and past (1982–2000) periods, increased recovery times for runoff and SWS were observed for all landcover types except for SWS (-0.4 month) in grasslands. However, recent recovery times for GPP and WUE were shorter than that in the past. Highly decreased recovery times for GPP (-2.2 months) and WUE (-2.4 months) were found in grasslands. Runoff recovery time increased by 4.9 months in forest and SWS recovery time increased by 1.4 months in cropland. These results improved our understanding of regional drought recovery mechanisms and future trends in different landcover types.

Spatiotemporal characterization of droughts and vegetation response in Northwest Africa from 1981 to 2020

Drought has become one of the most devastating natural risks of agricultural production and the environment in almost all climate regions. Thus, understanding the spatiotemporal characteristics of drought and its associated impacts is crucial in drought early warning management and adaptation efforts. In this study, we used the 3-month Standardized Precipitation Index (SPI-3) obtained from Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) to investigate the space–time characteristics of drought conditions. Also, this study examined the impact of the SPI-based drought on vegetation health conditions using the Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) time-series data from 1981 to 2020. The results revealed that the region experiences drought primarily between July and September, with the most prolonged drought events lasting up to five months. Morocco suffered from more frequent droughts than other countries in the region. The Mann-Kendall test showed that the trend of drought became drier over the last decade, whereas the period from 1981 to 2010 witnessed either wetting or no trends. This study also found that the response of crops and grasslands showed higher correlation with the SPI-3 and that the response of vegetation to droughts was higher during the dry season. The findings of this study provide useful information to support local and regional drought planning and adaptation programs and enhance the understanding of drought development in the region.

Evaluation of the impact of large-scale atmospheric indicators and meteorological variables on drought in different regions of Iran

Evaluation of the impact of large-scale atmospheric indicators and meteorological variables on drought in different regions of Iran

Human activities impact the propagation from meteorological to hydrological drought in the Yellow River Basin, China

Quantifying the role of human activities in the process of drought propagation from meteorological drought to hydrological drought is helpful in developing mitigation strategies and in prediction of hydrological drought. To quantify the role of human activities, we compared the results from using simulated natural and observed streamflow to calculate drought indices and drought propagation time. In addition, we compared different propagation detection methods for evaluating the spatiotemporal characteristics of drought propagation from meteorological to hydrological in the Yellow River Basin during the period 1962–2018. The results show: (1) The intensity of meteorological drought (characterized by the standardized precipitation evapotranspiration index) in upstream regions showed an upward trend on the short-term (1-month) scale. At the 6-month scale, human activities reduced hydrological drought events (characterized by the standardized streamflow index) at 93% of stations in the midstream and downstream reaches. (2) The proposed center of gravity method is more suitable for calculating the propagation time after the impact of human activities, while the onset method is suitable for early drought warning. (3) Human activities shortened the propagation time for the controlled area of about 58% of stations (11 out of 19) during the growing season and prolonged it for the controlled area of about 53% of stations (10 out of 19) during the nongrowing season in the Yellow River Basin.

Normalized Temperature Drought Index (NTDI) for Soil Moisture Monitoring Using MODIS and Landsat-8 Data

As the fundamental regulator of energy exchange in the vegetation–soil–atmosphere circulation system, soil moisture is a key parameter for drought monitoring and is indispensable to the land surface hydrological processes. In order to overcome the constraints of the Perpendicular Drought Index, PDI (performs poorly over the fields with dense vegetation and hard to construct the soil line), and the Temperature Vegetation Drought Index, TVDI (requires similar atmospheric forcing and large enough dimension of mapping area), in soil moisture monitoring, a new drought index (Normalized Temperature Drought Index, NTDI) is proposed to explore the spatiotemporal changes of soil moisture by substituting red and near-infrared reflectances with vegetation index and normalized land surface temperature on the basis of the PDI framework. Victoria, Australia, was selected as the study area as it experiences many severe droughts and has been affected for more than ten years. Time series of satellite-based data were applied to evaluate the effectiveness and applicability of the NTDI at the regional scale. Results indicated that the expression of the soil line representing the water condition of the bare soil is easier to obtain in the new trapezoid framework and has good fits with the coefficients of determination (R2) of more than 0.8. Compared with PDI, TVDI and Modified PDI (MPDI) at the cropping sites, NTDI exhibits a relatively better performance in soil moisture monitoring for most days where the R2 achieved can reach to more than 0.7 on DOY 242, 254 and 272. Meanwhile, spatial–temporal mappings of the four drought indices from satellite data were conducted, and the NTDI presented the slightly seasonal variation and effectively described the real spatial characteristics of regional drought. Overall, the NTDI seems to a viable approach and can provide insight into spatial and temporal soil moisture monitoring at different scales.

Responses of ecosystem water use efficiency to drought in the Lancang–Mekong River Basin

The ecosystem water use efficiency (WUE), a crucial indicator of how climate change will affect terrestrial ecosystems, depicts the coupling of the carbon gain and water loss in terrestrial ecosystems. In this study, the spatiotemporal variations in the WUE and its responses to drought in the Lancang–Mekong River Basin (LMRB) from 1982 to 2018 were investigated using the gross primary productivity (GPP) and evapotranspiration (ET) data acquired from the Global Land Surface Satellite (GLASS) products. The analyses revealed that: (1) the mean yearly WUE for the LMRB was 1.63 g C kg−1 H2O, with comparatively higher values in forests and warm temperate climatic types. The interaction of temperature and leaf area index was the main factor affecting the spatial distribution of WUE. The yearly WUE for the entire region exhibited a decreasing trend with a rate of −0.0009 g C kg −1 H2O·yr−1, and the spatially significantly decreasing area accounted for 41.67% of the total area. (2) The annual WUE was positively correlated with drought in the humid regions, accounting for 66.55% of the total area, while a negative relationship mainly occurred in the high-altitude cold region. (3) The ecosystem WUE lagged behind the drought by 3 months in most regions. The lag effect was more apparent in the grassland-dominated upstream region and the cropland-dominated Mekong Delta. (4) The resilience analysis revealed that the ecosystems in forests and temperate climate types were strongly resistant to drought, while the grassland and high-altitude regions with a dry and cold climate had relatively poor resilience. The results of this study shed light on how the WUE responds to drought across diverse land use types, climate types, and elevation gradients, uncovering fresh insights into the potential mechanisms behind the impact of drought on water and carbon cycles within ecosystems.

Storage infiltration of rock-soil interface soil on rock surface flow in the rocky desertification area

Rock surface flow, formed from the rock surface of outcrops after receiving rainwater, has become a main form of runoff in rocky desertification ecosystems with frequent regional and seasonal droughts. However, as a main flow path of rock surface flow into the surrounding soil of outcrops, how much the rock-soil interface soil can store and infiltrate the rock surface flow from outcrops is not well known. The goals of this study were to explore the storage-infiltration effect of soil at the rock-soil interface on rock surface flow in rocky desertification areas. Hence, a typical rocky land with large bulk rock outcrops, of which the surrounding soil has experienced five years of rock surface flow, and three special rock surfaces with shapes of convex, straight, and concave with different hydrological paths were selected in a typical rocky desertification county of China. Field investigation, ring knife soaking and infiltration methods were applied to indicate the difference in storage-infiltration between rock-soil interface soil and non-rock-soil interface soil on the rock surface flow in these different rock surface shapes. The results showed that the rock-soil interface soils around outcrops with different rock surface shapes exhibited different storage-infiltration capabilities for rock surface flow. The space of soil carrying water at the rock-soil interface showed the order of the convex shape, the straight shape and the concave shape, in which the non-rock-soil interface soil around the convex shape had greater space, followed by the rock-soil interface soil. The rock-soil interface soil of all three shapes had higher water storage performance than the non-rock-soil interface soil in terms of the field capacity and usable storage, where the rock-soil interface soil at the soil depth of 0–10 cm of the concave shape had lower water storage and lower conservation capacity than the 10–20 cm depth, and this was of positive significance for retaining the rock surface flow. The soil infiltration rate at the rock-soil interface with only the concave shape was greater than that at the non-rock-soil interface, where the concave shape had the greatest soil infiltration rate at the rock-soil interface. The results from this study can be used to understand the influence of outcrops on the soil properties of karst ecosystems.