Implications of Hysteresis on the Horizontal Soil Water Redistribution after Infiltration

Although soil water redistribution is critical for a number of problems, a rather limited study of this process has been reported up to now and especially as regards the implications of hysteresis on horizontal soil water redistribution after infiltration. To this end, a thorough theoretical and numerical investigation of the redistributed soil water content profiles formed after the cessation of a horizontal infiltration is presented. A number of different initial soil water contents before the initiation of the horizontal infiltration and different infiltration depths were analyzed using the HYDRUS-1D software package considering the appropriate hysteretic wetting and drying curves. The effect of neglecting hysteresis was also investigated for the same conditions. The main wetting and drying boundary curves of the studied porous medium and the hydraulic conductivity at saturation were experimentally determined. The theoretical and numerical analysis indicated that the form of the redistributed soil water content profiles in the presence of hysteresis was similar to the original infiltration profile independently of whether the initial soil water content was taken on the boundary wetting or drying curve and independently of the porous medium type. Specifically, in a relatively short time after the initiation of the redistribution process, the magnitude of the soil matrix head gradient tended to zero due to hysteresis, and this resulted in an insignificant soil water movement, although the soil water content and the hydraulic conductivity values were still high. In addition, the redistribution proceeded at a faster rate than the smallest depth of infiltration water prior to the redistribution, and it was faster during the early stages of the redistribution. Accordingly, hysteresis is important for the simulation of horizontal soil water redistribution as it is, for example, in the case of localized irrigation systems’ design and management.

Measurement of unsaturated meltwater percolation flux in seasonal snowpack using self-potential

This paper presents a feasibility study of in situ field measurements of unsaturated meltwater percolation flux within the vertical profile of a snowpack, using the self-potential (SP) method. On-site snowmelt column tests calibrated the SP measurements. The SP data measured electrical field strength with an electrode spacing of 20 cm, and coincident water saturation (Sw) measurements using time domain reflectometry allowed calculation of SP-modeled vertical percolation flux (qsp), expressed as Darcy velocity. The results reflected transient diurnal snowmelt dynamics, with peak flux lagging arrival of a saturation wetting front. Peak daily qsp was 60 to >300 mm d−1, whereas daily snowmelt was 20–50 mm w.e. Surface refreezing events appeared to cause upward flow, possibly representing water redistribution toward the freezing boundary. Calculated fluxes were comparable to actual fluxes, although average errors ranged from −15 to +46% compared to average of melt expected from surface energy-balance and ablation stake measurements. By advancing method development to measure unsaturated meltwater percolation flux in snowpacks this study creates opportunities to study fundamental snowmelt processes, may improve mathematical modeling and may supplement glacier mass-balance studies and studies of snowmelt interactions with avalanches, groundwater and surface water.

Open areas in patchy ecosystems: key spaces for vegetation survival.

<p>Drylands are one of the largest biomes over the Earth, covering around 40% of land surface. These are water limited ecosystems where vegetation occupies the most favourable positions over the landscape. Less favourable areas are frequently covered by other biotic and abiotic components such as biological soil crusts, bare soil, or stones. During most rainfall events, runoff is generated in open areas (runoff sources) and redistributed through vegetation patches (runoff sinks), therefore increasing water and nutrient availability for plants. Water redistribution feedbacks determine vegetation coverage and productivity, modulate changes in its spatial distribution, and could ameliorate the predicted negative effects of climate change over these ecosystems.</p><p>The principal aim of this study was to quantify the impact of water redistribution processes on vegetation performance, and to evaluate how this effect varies in response to aridity. To achieve it, we analysed the relationships between runoff redistribution from open areas and vegetation productivity, by combining satellite information on vegetation state and topography. More precisely, we calculated Normalized Difference Vegetation Index (NDVI) dynamics during three hydrological years in 17 study sites along an aridity gradient in the SE of the Iberian Peninsula using SENTINEL 2 images. Then we used a DEM and a high spatial resolution vegetation map to derive a water redistribution index that simulate source-sinks interactions between vegetation and open areas. Finally, we analyse the relationship between, potential water redistribution and vegetation dynamics and how it varies along the aridity gradient.</p><p>We found a non-linear relationship between potential water redistribution and vegetation productivity. Overall, vegetation NDVI increases as potential water redistribution did, which demonstrated the importance of water redistribution processes on drylands vegetation performance. However, vegetation capacity to retain runoff water is limited and there is a clear threshold above which increased potential water redistribution does not promote vegetation productivity. Thresholds are caused by the limit capacity of vegetation to infiltrate run off when preferential flows are forming, increasing ecosystem connectivity, and involving local water losses for vegetation.  Therefore, an increase in open areas between vegetation patches could have a positive effect over vegetation through hydrological connectivity but until to a certain point in which global connectivity supposed water losses for plants. This process could have important effects under climate change, by controlling the resistance and resilience of vegetation in drylands ecosystems.</p><p>Acknowledgements. This research was supported by the FPU predoctoral fellowship from the Educational, Culture and Sports Ministry of Spain (FPU17/01886) REBIOARID (RTI2018-101921-B-I00) projects, funded by the FEDER/Science and Innovation Ministry-National Research Agency, and the RH2O-ARID (P18-RT-5130) funded by Junta de Andalucía and the European Union for Regional Development.</p>

Global flood monitoring with GRACE/GRACE-FO

<p>The German Aerospace Center and NASA's joint mission, the Gravity Recovery and Climate Experiment (GRACE) operational from 2002 until October 2017, provided measurements of Earth's gravity field anomalies. Its follow-on mission GRACE-FO, implemented by NASA and GFZ, was launched in May 2018 and continued to give us large-scale measurements of the Earth's gravity variations. These variations in gravity are used to determine anomalies of total water storage (TWSA) which can provide us with insights into global water redistribution on a monthly up to a daily basis.</p><p>Most common natural disasters that still require efficient early warning systems are floods. Floods are causing significant economic and humanitarian losses on a global scale and are triggered by the interaction of different hydro-meteorological processes (e.g. precipitation, sub-surface water storage, snow cover).    </p><p>We aim to explore GRACE and GRACE-FO products' possibilities to detect the water storage dynamics associated with floods in large river catchments. We include analysis of the basins' wetness states before the flood events, which eventually can give us early indicators of flood development. During the GRACE data period, we investigate around 2500 historical floods from the Dartmouth Flood Observatory (DFO). We acquire GRACE data with daily resolution from the latest releases of ITSG and GFZ for the spatial extent of DFO floods and reduce TWSA values by long-term trends and by average seasonal variability. Furthermore, we assess the available river discharge time series, during the GRACE period, obtained from the Global Runoff Data Centre (GRDC) for the flood event separation. We compare GRACE-based water storage anomalies to flood events' characteristics, like peak, volume, and duration. Results show the potential of GRACE-based TWSA to detect large-scale flood events.</p>

Polystructural organization of a landscape: geophysical analysis of geosystems synergy

Эмпирические концепции ландшафтоведения в рамках геофизической парадигмы позволяют ассимилировать физические законы для описания структуры и функционирования геосистем. Исследованы южно-таежные ландшафты конечно-моренной зоны Валдайского оледенения на территории Национального парка «Валдайский». Геоструктуры описываются с помощью параметров силовых геофизических полей – поля силы тяжести и поля инсоляции, которые можно получить на основе морфометрии цифровых моделей рельефа и цифровых данных космической съемки. Выбор небольшого числа главных параметров с ясным физическим смыслом проводится в соответствии с классическими определениями. Обоснование числа и значимости параметров состояния элементарных пикселей и геосистем осуществляется при анализе результатов численного моделирования структуры ландшафтов. Методом дихотомической группировки элементов поверхности рельефа по параметрам состояния (высота, уклон, горизонтальная и вертикальная кривизна, доза прямой солнечной радиации, NDVI) реализован типологический подход к классификации природно-территориальных комплексов (по принципу однородности) и получена структура ландшафтов на уровне урочищ. С другой стороны, функциональный подход позволил построить иерархию водосборных геосистем по морфометрическим параметрам, описывающим перераспределение воды в поле гравитации – уклонам, удельной площади водосбора, горизонтальной и вертикальной кривизне. Все классификации строятся на единой базе данных и могут использоваться для разных прикладных задач. Приводится пример расчета скоростей стока поверхностных вод и на их основе – зонирование водосборных геосистем по времени добегания до контрольных створов. Таким образом, формальный алгоритм выделения наименьших и иерархических единиц поверхности рельефа на основе параметров состояния приобретает фундаментальный геофизический смысл. Понятие полиструктурности ландшафта в этом случае является абсолютно логичным: выбирая те или иные физически содержательные структурообразующие процессы и их параметры, можно реализовать разные классификации ландшафтов, необходимые в прикладных задачах. Empirical concepts of physical geography enable us to apply the physical laws to describe the landscape structure and functioning. The finitely morainic landscapes of the Valdai Glaciation on the National Park “Valdaisky” territory were investigated. A development of geostructures identified by classical landscape analysis can be described by the parameters of power geophysical fields, mainly gravitation and insolation fields. Selection of a small number of main parameters with extremely clear physical meaning is carried out in accordance with the classical definitions of landscape science. Justification of the number and importance of parameters of elementary pixels and geosystems is carried out when analyzing the results of numerical simulation of the structure of landscapes. Using the method of dichotomic grouping of the relief surface elements by the state parameters (height, slope, horizontal and vertical curvature, dose of direct solar radiation, NDVI), the typological approach to the classification of the natural-territorial complexes (according to homogeneity principle) was realized and the structure of landscapes at a level of natural boundaries was obtained. On the other hand, the functional approach allowed to construct the hierarchy of the water-collecting geosystems by morphometic parameters describing the water redistribution in the gravitation field – slopes, drainage factor, horizontal and vertical curvature. All the classifications are constructed on the single base of data and can be used for different applied tasks. An example of calculating the surface water flow rate is presented and, on their basis, a zoning of the water-collecting geosystems, using the flow time to the control sections. If one uses these parameters the formal mathematical algorithm for identification of elementary and hierarchical units of landforms acquires fundamental geophysical interpretation. In this case the concept of landscape patterns multiplicity is quite relevant. By choosing these or other physical parameters and structure-forming processes we have the opportunity to perform various landscape classifications needed in the applied tasks.