Preface: Linking landscape organisation and hydrological functioning: from hypotheses and observations to concepts, models and understanding

The link between landscape properties and hydrological functioning is the very foundation of hydrological sciences. The fundamental perception that landscape organisation and its hydrological and biogeochemical processes co-develop is often discussed. However, different landscape characteristics and hydrological processes interact in complex ways. Hence, the causal links between both are usually not directly deducible from our observations. So far no common concepts have been established to connect observations, properties and functions at and between different scales. This special issue hosts a broad set of original studies indicating the current state and progress in our understanding of different facets of dynamic hydrological systems across various scales. It is organised as a joint special issue in HESS and ESSD, with the purpose of providing the scientific insights in combination with the underlying data sets and study design. While the individual studies contribute to distinct aspects of the link between landscape characteristics and hydrological functioning, it remained difficult to compile their specific findings to more general conclusions. In this preface, we summarise the contributions. In the search for ways to synthesise these individual studies to the overall topic of linking landscape organisation and hydrological functioning, we suggest four major points how this process could be facilitated in the future: (i) formulating clear and testable research hypotheses, (ii) establishing appropriate sampling designs to test these hypotheses, (iii) fully providing the data and code, and (iv) clarifying and communicating scales of observations and concepts as well as scale transfers.

Patterns and drivers of evapotranspiration in South American wetlands

Evapotranspiration (ET) is a key process linking surface and atmospheric energy budgets, yet its drivers and patterns across wetlandscapes are poorly understood worldwide. Here we assess the ET dynamics in 12 wetlands complexes across South America, revealing major differences under temperate, tropical, and equatorial climates. While net radiation is a dominant driver of ET seasonality in most environments, flooding also contributes strongly to ET in tropical and equatorial wetlands, especially in meeting the evaporative demand. Moreover, significant water losses through wetlands and ET differences between wetlands and uplands occur in temperate, water-limited environments and in highly flooded areas such as the Pantanal, where slow river flood propagation drives the ET dynamics. Finally, floodplain forests produce the greatest ET in all environments except the Central Amazon, where upland forests sustain high rates year round. Our findings highlight the unique hydrological functioning and ecosystem services provided by wetlands on a continental scale.

Changes in soil hydraulic conductivity and preferential flow pathways after assisted forest restoration on degraded land in the Khasi Hills (Meghalaya, NE India)

<p>As in other parts of the Indian subcontinent, the forests of Meghalaya (NE India) provide an array of environmental services but the prevalence of traditional slash-and-burn agriculture (locally called jhum) plus mining has led to severely degraded hillsides and a critical loss of soil water storage opportunity and groundwater recharge. As a result, despite receiving some of the highest rainfall totals in the world (MAP up to 11 m, 75% received between May and September), the Meghalaya plateau faces severe water scarcity during the five-month long dry season. In response to such problems, initiatives have been taken towards restoring hydrological functioning through reforestation and assisted natural regeneration (ANR) programmes. As a first step towards assessing the possible improvement of soil physical characteristics and associated hydrological functioning after several years of ANR we measured topsoil (0–10 cm) saturated hydraulic conductivity (K<sub>sat</sub>) using double-ring infiltrometry at 12 sites in the Khasi Hills that represented three contrasting vegetation covers: (i) sacred forest (n = 6, natural baseline), (ii) 2–10-year-old ANR (n = 3), and (iii) degraded Imperata grassland (n = 3, degraded reference). At each site, nine K<sub>sat</sub>-measurements were taken along the hillslope gradient. In addition, at three sites, blue dye infiltration experiments (n = 2 per site) were carried out to examine the dominant percolation pathways associated with each land-cover type. The median K<sub>sat</sub> value for the sacred forest sites was highest (373 mm h<sup>-1</sup>), reflecting the abundance of biologically mediated macropores arising from the decomposing activity of soil microflora and fauna at these relatively undisturbed sites. The corresponding value for the ANR sites (160 mm h<sup>-1</sup>) was much higher than the median K<sub>sat</sub> for the degraded grasslands (71 mm h<sup>-1</sup>) but still considerably below the forest reference. Limited observations of topsoil bulk density and carbon content (n = 5 samples in each of three plots) showed increasing bulk density and decreasing carbon content from forest via ANR to grassland, thereby reflecting the observed trend in K<sub>sat</sub>. The blue dye experiments suggested infiltration in the sacred forest was dominated by flow along roots and other preferential flow pathways whereas the degraded grassland was mostly characterized by matrix flow. The ANR site showed intermediate behaviour with macropore flow exhibiting high matrix interaction. Comparison of observed median topsoil K<sub>sat</sub> in top-layer with prevailing (maximum) hourly rainfall intensities for Cherrapunji suggested infiltration-excess overland flow (IOF) must be considered a rare phenomenon in the sacred forest. Conversely, the K<sub>sat</sub>-values for the ANR and degraded grassland sites indicated the occurrence of IOF at high-intensity rainfall events. Despite the observed improvement in surface K<sub>sat</sub> it cannot be excluded that the generally shallow nature and high stoniness of the soils pose serious limitations to rebuilding soil water storage capacity through ANR/reforestation. Furthermore, frequent occurrence of saturation-excess OF at the height of the monsoon and associated surface erosion cannot be excluded.</p>

Mediterranean Temporary Ponds: using isotope hydrology tools to describe and understand their behaviour

<p>Mediterranean temporary ponds are very shallow ponds, isolated from permanent water bodies, which undergo a periodic cycle of flooding and drought, and have a characteristic flora and fauna adapted to this alternation. This habitat is mainly distributed in dry and sub-arid areas. Mediterranean temporary ponds are identified as one of the worldwide biodiversity hotspots and constitutes therefore a priority habitats according to the Natura 2000 network of the European Union (3170*, Council Directive 92/43/CEE). The development of flora and fauna in this type of ecosystem is defined by the natural length of the hydro-period. However, little is known about the hydrological functioning of these very specific hydrosystems. DespiteHS10 this protective conservation status, this habitat has suffered continuous degradation and loss disappearing at a fast rate due anthropogenic impacts and climate pressures. In most cases, temporary wetland disappearance is unintentional and related to a lack of understanding of its hydrological functioning within the watershed.</p><p>The aim of this work is, hence, to use the tools of the isotope hydrology to increase our basic understanding of the hydrological functioning of the Mediterranean temporary ponds. Our study focuses on the Musella temporary pond located in Southern Corsica (France) which undergoes important man-induced and climatic pressures. During one full hydrological cycle, surface and groundwater levels, major ions, stable isotopes of the water molecules as well as field parameters (temperature, pH, electrical conductivity, dissolved oxygen) have been measured every month.</p><p>Results bring information on the water quality, chemical stability and temporal evolution in terms of surface water level as well as potential connection with the underlying carbonated aquifer. The stable isotopes inform about the origin of water, its mixing processes with groundwater, and its evaporative status through time.</p><p>Flooding and drying processes of the Musella temporary pond are now better constrained and documented projections can now be set up towards the resilience of the hydrosystem considering the future consequences of climate change in the Mediterranean region.</p>

Proposal of a typology of karst systems functioning based on relevant indicators of karst springs hydrodynamics

<p>10% of the world’s population is dependent on karst water resources for drinking water. Understanding the functioning of these complex and heterogeneous systems is therefore a major challenge for long term water resource management. Over the past century, different methods have been developed to analyse hydrological series, and subsequently characterize the functioning of karst systems. These methods can be considered as a preliminary step in the development and design of hydrological models of karst functioning for sustainable water resource management. Recent progress in analytical tools, as well as the emergence of data bases of discharge time series (e.g. the French SNO KARST database and the WoKaS database at global scale) allow reconsidering former typology of karst system hydrodynamic responses. Ten karst systems and associated spring discharge time series were considered for developing the typology. The systems are well-known with a high-quality monitoring and they cover a wide range of hydrological functioning, which ensure the relevance of the analyses. The methodology for the assessment and the development of the typology consisted in (i) the analysis of springs discharge time series according to four different methods, (ii) the selection or proposal of the most relevant indicators of karst systems hydrodynamics, and (iii) the interpretation of the information from these indicators based on principal component analysis and clustering techniques. A typology of karst systems accounting for 6 different classes is finally proposed, based on 3 aspects of functioning: the capacity of dynamic storage, the draining dynamic of the capacitive function and the variability of the hydrological functioning. The typology was applied to a wider dataset composed of spring discharge of 78 karst systems. The results show a relevant distribution of the systems among the different classes.</p>

Climate change impact on the hydrological functioning of the mountain lakes: a conceptual framework

<p>Mountain lakes are distinctive water bodies that not only serve as a crucial water resource for the inhabitants of the upland regions but also as an important destination for millions of tourists who are attracted by their beauty. Mountain lakes are fragile water bodies that are experiencing changes in their hydrological processes owing to global warming. Understanding the consequences is important as it can help identify whether climate change causes degradations in lake hydrological functioning. The interactions of hydrological processes in mountain lakes with external drivers are usually hard to explain explicitly owing to their complexity. To deal with that problem, scholars develop conceptual frameworks. The focus was on the Canadian Rocky Mountains where 5155 lakes were identified using GIS. To identify factors influencing lake hydrological function and their sensitivity to changing climate, a literature review was undertaken. Identified in the literature review were 13 natural drivers that reflected climate change impacts to lake hydrological functioning and 38 additional sub-factors that characterize the drivers. Using these factors, a conceptual framework for mountain lake hydrological functioning was developed. The major limitation to thorough testing of the conceptual framework was a small number of observations for lakes in the research area. Nevertheless, the conceptual framework is flexible and should be tested across many mountainous regions worldwide. Overall, the conducted research stresses the problem of limited hydrological understanding of lakes in the Canadian Rockies and presents a useful framework of the complex interactions of natural drivers and intra-lake processes under rising temperatures.</p>