Integrating the Budyko framework with the emerging hot spot analysis in local land use planning for regulating surface evapotranspiration ratio

Land use planning regulates surface hydrological processes by adjusting land properties with varied evapotranspiration ratios. However, a dearth of empirical spatial information hampers the regulation of place-specific hydrological processes. Therefore, this study proposed a Local Land Use Planning framework for EvapoTranspiration Ratio regulations (ETR-LLUP), which was tested for the developments of spatially-varied land use strategies in the Dongjiang River Basin (DRB) in Southern China. With the first attempt at integrating the Emerging Hot Spots Analysis (EHSA) with the Budyko framework, the spatiotemporal trends of evapotranspiration ratios based on evaporative index and dryness index, from 1992 to 2018, were illustrated. Then, representative land-cover types in each sub-basin were defined using Geographically Weighted Principal Component Analysis, in two wet years (1998 and 2016) and three dry years (2004, 2009, and 2018), which in turn were identified using the Standard Precipitation Index. Finally, Geographically Weighted Regressions (GWRs) were used to detect spatially-varied relationships between land-cover proportions and evaporative index in both dry and wet climates. Results showed that the DRB was consistently a water-limited region from 1992 to 2018, and the situation was getting worse. We also identified the upper DRB as hotspots for hydrological management. Forests and croplands experienced increasingly water stress compared to other vegetation types. More importantly, the spatial results of GWR models enabled us to adjust basin land use by 1) expanding and contracting a combination of ‘mosaic natural vegetation’ and ‘broadleaved deciduous trees’ in the western and eastern parts of the basin, respectively; and 2) increasing ‘broadleaved evergreen trees’ in the upstream parts of the basin. These spatially-varied land use strategies based on the ETR-LLUP framework allow for place-specific hydrological management during both dry and wet climates

Quantifying land use heterogeneity on drought conditions for mitigation strategies development in the Dongjiang River Basin, China

Spatially-invariant land use and cover changes (LUCC) are not suitable for managing non-stationary drought conditions. Therefore, developing a spatially varying framework for managing land resources is necessary. In this study, the Dongjiang River Basin in South China is used to exemplify the significance of spatial heterogeneity in land planning optimization for mitigating drought risks. Using ERA5 that is the 5th major atmospheric reanalysis from the European Centre for Medium-Range Weather Forecast, we computed the Standardized Runoff Index (SRI) to quantify the hydrologic drought during 1992 to 2018. Also, based on Climate Change Initiative land use product, The Geographically Weighted Principal Component Analysis was used to identify the most dominant land types in the same period. Then, we used the Emerging Hot Spots Analysis to characterize the spatiotemporal evolution of historical LUCC and SRI. The spatially varying coefficients of Geographically and Temporally Weighted Regression models were used to reveal the empirical relationships between land types and the SRI. Results indicated that rainfed cropland with herbaceous cover, mosaic tress and shrub, shrubland, and grassland were four land types having statistical correlations with drought conditions over 27 years. Moreover, since 2003, the DRB was becoming drier, and the northern areas generally experienced severer hydrologic drought than the south. More importantly, we proposed region-specific land-use strategies for drought risk reductions. At a basin scale, we recommended to 1) increase rainfed herbaceous cropland and 2) reduce mosaic tree and shrub. At a sub-basin scale, the extents of shrub and grassland were suggested to increase in the northern DRB but to reduce in the south. Region-specific land use planning, including suitable locations, scales, and strategies, will contribute to handling current ‘one-size-fits-all’ LUCC. Planners are suggested to integrate spatial characteristics into future LUCC for regional hydrologic management

The grass is not always greener on the other side: Seasonal reversal of vegetation greenness in aspect-driven semiarid ecosystems

Our current understanding of semiarid ecosystems is that they tend to display higher vegetation greenness on polar-facing slopes (PFS) than on equatorial-facing slopes (EFS). However, recent studies have argued that higher vegetation greenness can occur on EFS during part of the year. To assess whether this seasonal reversal of aspect-driven vegetation is a common occurrence, we conducted a global-scale analysis of vegetation greenness on a monthly time scale over an 18-year period (2000–2017). We examined the influence of climate seasonality on the normalized difference vegetation index (NDVI) values of PFS and EFS at 60 different catchments with aspect-controlled vegetation located across all continents except Antarctica. Our results show that an overwhelming majority of sites (70%) display seasonal reversal, associated with transitions from water-limited to energy-limited conditions during wet winters. These findings highlight the need to consider seasonal variations of aspect-driven vegetation patterns in ecohydrology, geomorphology, and Earth system models

Impact of the North Sea–Caspian pattern on meteorological drought and vegetation response over diverging environmental systems in western Eurasia

Emerging drought stress on vegetation over western Eurasia is linked to varying teleconnection patterns. The North Sea–Caspian Pattern (NCP) is a relatively less studied Eurasian teleconnection pattern, which has a role on drought conditions and the consequence of changing conditions on vegetation. Between 1981 and 2015, we found that the Standardized Precipitation Index (SPI) and the Normalized Difference Vegetation Index (NDVI) have different trend patterns over various parts of western Eurasia. Specifically, the vegetation greenness is linked with wetter conditions over Scandinavia, and vegetation cover decreases over a drying central Asia. However, western Russia and Franceare paradoxically becoming greener under drier conditions. Using the Budyko framework, such paradoxical patterns are found in energy-limited environmental systems, where vegetation growth is primarily promoted by warmer temperatures. While most studies focused on the impacts of the North Atlantic Oscillation (NAO), we test whether the NCP explains better the variability of meteorological drought and vegetation response over western Eurasia. We hypothesised that the positive phases of the NCP are correlated to high pressure anomalies over the North Sea, which can be associated with weakening onshore moisture advection, leading to warmer and dryness conditions. These conditions are driving vegetation greening, as western Eurasia is mainly energy limited. However, we show that as the climate is warming along with the teleconnection impacts, the future ecosystem over western Eurasia will be transferred from energy-limited to water-limited systems. This suggests that the observed vegetation greening over past three decades is unlikely to sustain in the future

Modeling the ecohydrologic role of solar radiation on catchment development in semi-arid ecosystems.

Thesis (Ph.D.)--University of Washington, 2014The role of solar radiation on ecohydrologic fluxes, vegetation dynamics, species composition, and landscape morphology has long been documented in field studies. However, these studies miss the value offered by a numerical modeling approach that integrates a range of ecohydrologic and geomorphic processes in exploring the landscape response to multiple controlling factors. This study represented flood generation and solar-radiation-driven echydrologic dynamics in a landscape evolution model (LEM) to investigate how ecohydrologic differences caused by differential irradiance on opposing hillslopes manifest themselves on the organization of modeled topography, soil moisture, and plant biomass. We use the CHILD (Channel-Hillslope Integrated Landscape Development) LEM equipped with a spatially-distributed solar-radiation component, leading to spatial patterns of soil moisture; a vegetation dynamics component that explicitly tracks above- and below-ground biomass; and a runoff component that allows for runoff-runon processes along landscape flow paths. This study starts with data analysis, and then followed by a modeling part. In the first part, the relationship between land surface properties (e.g. soil, vegetation, and lithology) and landscape morphology quantified by the catchment descriptors: the slope-area (S-A) relation, curvature-area (C-A) relation, and the cumulative area distribution (CAD), in two semiarid basins in central New Mexico. All three land surface properties were found to have significant influences on the S-A and C-A relations, while the power-law exponents of the CADs for these properties did not show any significant deviations from the narrow range of universal scaling exponents reported in the literature. Among the three different surface properties we investigated, vegetation had the most profound impact on the catchment descriptors. Following data analysis, the role of solar radiation on landscape morphology was investigated in the second part with a numerical model framework that integrated a range of ecohydrologic and geomorphic processes. Modeled spatial patterns of soil moisture confirmed empirical observations at the landscape scale as well as other hydrologic modeling studies. The spatial variability in soil moisture was controlled by aspect prior to the wet season (North American Monsoon, NAM), and by the hydraulic connectivity of the flow network during the NAM. Aspect and network connectivity signatures were also manifested on plant biomass with typically denser vegetation cover on north-facing slopes than south-facing slopes. Over the long-term, CHILD gave slightly steeper and less dissected north-facing slopes, more dissected south-facing slopes, and overall asymmetry in the modeled morphology of valleys. Aspect influence on hillslope asymmetry was enhanced with greater uplift rates. Model simulations showed how subtle differences in biomass and soil moisture dynamics at annual scales lead to distinct geomorphic differences at both hillslope and catchment scales. The controls of latitude and mean annual precipitation (MAP) on the development of hillslope asymmetry were investigated in the third part by using the CHILD LEM. In simulations the mean slope of north-facing slopes was steepened towards the poles, while south-facing slopes became gentler toward the poles. As a result of this inverse pattern, the relative differences between north- verses south-facing slopes become larger toward the poles. The model outcomes, which are compatible with field observations, show north-facing slopes to be steeper (shallower) than south-facing slopes in the northern (southern) hemisphere. Our results underscore the influence of solar radiation as a global control on the development of hillslope asymmetry. Variations in MAP at the same latitude have little impact on hillslope asymmetry in comparison to variations in latitude at the same MAP. In the last part, the observed spatial patterns in erosion rates caused by aspect-driven microclimatic and ecohydrologic conditions are examined with the CHILD LEM forced with a uniform uplift rate obtained by averaging the erosion estimates from the study site. Climate represented in the model ranges from simple to more realistic. The climate forcing is simulated by: (1) stationary climate represents the recent climate that prevails in the study site; (2) cyclic climate represents the late Pleistocene climate that prevailed in the region; (3) paleo-constructed climate based on paleoclimate proxies. Recent field study in central New Mexico shows that long-term erosion rates (~10,000 years) on south-facing slopes are faster than opposing north-facing slopes. However, CHILD simulations show that the discrepancy in erosion rates on opposing hillslopes is not sustainable over the long-term. Depending on the climate forcing or internal dynamics of erosion mechanism, either north- or south-facing slopes can be more erosive than their counterparts. Over the long-term, however, the fluctuations in spatial erosion rates are averaging out. Hence, under a given uniform uplift, erosion rates on opposing hillslopes are found to be the same

The merit of the North Sea - Caspian Pattern in explaining climate variability in the Euro-Mediterranean region

Teleconnection patterns are one of the key features of understanding high-frequency natural climate variability. The North Sea - Caspian Pattern (NCP) was identified as a middle tropospheric dipole and, its hydroclimatological implications were substantially restricted to the Eastern Mediterranean region. Thus, hydroclimatological influences of the NCP in the Euro-Mediterranean region were investigated in a comparative approach with dominant tropospheric teleconnections in the Eurasian region, and synoptic features such as ridge-trough positioning and strength. By using high-resolution ERA5 reanalysis data, cross-correlations between indexes, anticorrelations at 500 hPa, and composite anomaly maps for seasonally representative months were produced to understand the working mechanism of NCP. Comparisons included East Atlantic / Western Russian (EAWR), a rotated principal component analysis (RPCA) variant of NCP which utilizes pole-based representation Analysis revealed that NCP was correlated well with the Mediterranean trough displacement, and the strength of the East Asian trough. Climate anomalies indicated by NCP were greater and more spatially consistent compared to other teleconnections. NCP also had higher contrast of temperature and precipitation than EAWR based on the composite anomaly maps. In conclusion, NCP explained climate variability in all seasons linking remote centers of action within Eurasia's east and west extremes

Eco-geomorphic implications of hillslope aspect: Inferences from analysis of landscape morphology in central New Mexico

We investigate the influence of hillslope aspect on landscape morphology in central New Mexico, where differences in soils, vegetation, and landforms are observed between mesic north-facing and xeric south-facing slopes. Slope–area and curvature–area relations, derived from a Digital Elevation Model (DEM), are used to characterize the opposing hillslope morphologies. In all geologies and elevation ranges studied, topographic data reveal significantly steeper slopes in north-facing aspects, and shallower slopes in south-facing aspects. North-facing slope curvatures are also greater than south-facing curvatures. Using a conceptual slope-area model, we suggest that for a given drainage area, steeper north-facing slopes imply lower soil erodibility. We argue that this interpretation, consistent with recent views of ecosystem control on semiarid erosion rates, shows the influence hillslope aspect on topography and its associated vegetation communities. Observed valley asymmetry in the region reinforces this concept and suggests a long-term legacy of aspect-modulated ecogeomorphic processes

Revisiting Soil Water Potential: Towards a Better Understanding of Soil and Plant Interactions

Soil water potential (SWP) is vital for controlling the various biological and non-biological processes occurring through and across the soil-plant-atmosphere continuum (SPAC). Although the dynamics and mechanisms of SWP have been investigated for several decades, they are not as widely explored in ecohydrology research as soil moisture, due at least partly to the limitation of field observation methods. This limitation restricts the understanding of the responses of plant physiology and ecological processes to the SWP gradient and the ecohydrological functions of SWP dynamics in different contexts. Hence, in this work, we first briefly revisit the origin and development of the concept of SWP and then analyze the comprehensive factors that influence SWP and the improvement of SWP observation techniques at field scales, as well as strategies for developing new sensors for soil water status. We also propose views of focusing on the response characteristics of plant lateral roots, rather than taproots, to SWP dynamics, and using hormone signaling research to evaluate plant response signals to water stress. We end by providing potential challenges and insights that remain in related research, such as the limitations of the SWP evaluation methods and the future development direction of SWP data collection, management, and analysis. We also emphasize directions for the application of SWP in controlling plant pathogens and promoting the efficiency of resource acquisition by plants. In short, these reflections revisit the unique role of SWP in eco-hydrological processes, provide an update on the development of SWP research, and support the assessment of plant drought vulnerability under current and future climatic conditions

A framework to evaluate the accessibility, visibility, and intelligibility of green-blue spaces (GBSs) related to pedestrian movement

The planning of green-blue spaces (GBSs) requires considering the pedestrian needs in their walking routes for improving the walking experience. Incorporating the quantitative spatial characteristics of pedestrian movement is essential for pedestrian-friendly urban planning, which however received insufficient attention. Based on the space syntax theory, this study provided three indicators – accessibility, visibility, and intelligibility – to demonstrate the needs of physical access, visual access, and spatial cognition, respectively, in pedestrian movement. Measuring these three indicators, this study exemplified the planning of pedestrian-friendly GBSs using Guangzhou, China as a case study. Spatial design network analysis was used to quantify heterogeneous values of accessibility, visibility, and intelligibility of each GBS throughout the city. Moreover, we used principal component analysis to identify the leading indicators based on their weightings and then to calculate the scores to compare these three aspects of GBSs. The measurements of accessibility, visibility, and intelligibility of each GBS were then averaged across urban administrative districts for evaluating city-scale GBSs. The findings showed that GBSs in central districts were most accessible and visible but least intelligible. In contrast, the overall intelligibility of GBSs throughout the city was the greatest but the visibility was the least. Furthermore, intelligibility, as a more important factor than accessibility and visibility, should be particularly emphasized in future planning of pedestrian-friendly GBSs. Pedestrians from the central districts of Guangzhou city were most satisfied with the walking experience, in terms of accessing to, viewing, and cognizing the GBSs. ‘Yuexiu’, ‘Huadu’, and ‘Nansha’ districts were found as the key places where improved accessibility, visibility, and intelligibility were particularly needed to improve the GBS pedestrian-friendliness throughout the city. In summary, this study not only demonstrated a human-scale GBS evaluation framework for improving the human walking experience but also provided empirical evidence for building pedestrian-friendly green-blue spaces at the city scale