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Soil moisture measurements are very popular. Yet they provide a very incomplete picture about the state of the partially saturated zone and the capillary binding of soil water. Here we propose that the free energy of the soil water stock offers a superior perspective on storage dynamics, as it combines soil water content, gravity potential and matric potential data. Based on this new state variable, we show that the partially saturated zone is characterized by a system-specific balance of storage and release corresponding to local state of minimum free energy. The latter depends on the soil water retention curve and topography/depth to groundwater. In the absence of an external rainfall or radiative forcing, the system will thus naturally relax back to and persist in this equilibrium. Hydrological systems are however not isolated, they are frequently forced out of their equilibrium either by rainfall or by radiation driven evaporation. Here we show that the corresponding storage dynamics manifests as deviations of the free energy from and relaxations back the local equilibrium and that the latter separates two different regimes, which are either associated with a storage excess and overshoot of potential energy or a storage deficit and overshoot of capillary binding energy. We demonstrate that these pseudo oscillations are distinctly different in different hydrological landscapes. As the free energy state of the soil water stock, the storage equilibrium and the ranges of both storage regimes depend jointly on depth to groundwater and the soil water retention curve, we combine both controls into a hydrological system characteristics we call the ‘energy state function’ of the soil. We show that the latter allows an insightful inter-comparison of storage dynamics with in different hydrological landscapes, and a priory estimate of depth to groundwater, based on available soil moisture and matric potential data. Finally, we demonstrate a threshold-like relation between the free energy of soil water in the riparian zone and streamflow generation, where the tipping points coincides with the transition from a storage deficit to a storage excess.

The understanding of soil hydrological processes in agroforestry systems has increased in recent years. However, key aspects determining the successful functionality of agroecosystems (e.g. plant-water availability, nutrient supply) are influenced by many factors and are therefore challenging to generalize. Such information is critical for management and planning strategies. If dominant processes such as evapotranspiration and the related controls on the soil water stock can be represented adequately in hydrological models at the plot scale, they can provide useful insights for practitioners.Here, we tested whether the physically-based CATFLOW model is capable of reproducing soil moisture dynamics in a South African agroforestry system consisting of windbreaks and irrigated blackberry plants. We initialised the model with matric potential measurements and calibrated it to soil moisture dynamics at two locations with differing vegetation within the test site. After successful calibration, several numerical experiments were performed to shed light on the presence and absence of windbreaks and of different irrigation strategies on the seasonal dynamics of plant available soil water at the test site. The model and observations were also compared in the frequency domain by using empirical mode decomposition as an additional model verification.The measured soil moisture time series are dominated by a gradual drying of the soil throughout the summer, which is less pronounced in the deeper soil ranges, while several rain events interrupted this general pattern. The model captured the drying as well as the amplitudes of the rain peaks well. However, there was an offset between measured and modelled absolute values due to the initiation based on matric potential. The simulated water balance revealed distinct differences between the windbreak and berry rows due to differences in rain interception, and evapotranspiration or irrigation patterns. Similarities in the frequency spectrum of observed and modelled values were apparent. On the other hand, the modelled time series showed more distinct spectra and less noise. Currently, more detailed analyses are being carried out to extract information from the frequency spectra.

Agroforestry systems (AFSs) are associated with many benefits such as augmented soil fertility or enhanced biodiversity. However, in water-limited areas the competition over water resources between trees and crops can reduce the productivity of the crop component. We want to share some of our results gained from in-depth analyses of time series (October 2019 to March 2020) and campaign (September 2019) data of soil moisture and matric potential in a South African AFS.Soil water content was measured in a soil profile at two locations within an AFS plot: alongside a windbreak consisting of Italian Alders (Alnus cordata) and amongst the crop i.e. within blackberry rows. Matric potential time series are only available at the windbreak soil profile. Surficial soil samples taken along transects perpendicular to the windbreak were analysed for physical properties (e.g. texture, water retention curve).Based on extracted water retention curves and matric potential time series, we found no evidence for plant water limitation during the measurement period (summer months) within the field site. Estimated root water uptake indicated that the trees take water from a greater range of depths, including deeper layers, than the blackberry plants. We observed divergent hydrological behaviour of the soil at the two locations during precipitation events, potentially resulting from dissimilar storage capacities and runoff formation potentials. Furthermore, the matric potential revealed hydrological information on plant water usage that was not as obvious from the soil moisture data.

Water limitation provides the potential to hinder the productivity of agricultural systems especially in arid and semi-arid regions. In agroforestry systems interactions between trees and crops range from mutually beneficial to critically competing, shaping the demand for resources, such as water. In this study, we investigated the hydrological effects of an Italian Alder (Alnus cordata) windbreak on an irrigated blackberry plantation near Stellenbosch, South Africa. We determine the key components of the water budget in the system and compare them at two positions: alongside the windbreak, and amongst the crop away from the windbreak’s influence.We measured soil water content depth profiles in the summer months, from October 2019 to March 2020, in both locations with four consecutive time domain reflectometry (TDR) tube sensors, each integrating over 20 cm depth. Potential evapotranspiration (ET) was estimated from site based meteorological observations. We surveyed and classified the local soil, and defined soil chemical and physical properties (e.g. texture, matrix potential). The windbreak structure was measured on a single tree basis (e.g. tree height, volume and biomass) using manual and terrestrial laser scanning methodologies.The data indicate that high potential ET, caused by high summer temperatures and strong winds, dominates the water budget at the study site, exceeding the water input of the drip irrigation. We found differences in the water dynamics between the two sites, e.g. greater soil water content at greater distances from the windbreak. Possible reasons are: (1) the water demand of trees increases underground competition for water, and/or; (2) microclimatic conditions closer to the windbreak increase ET. Modelling of the windbreak influence on the ET and further analysis of water fluxes will be conducted as next steps to combine the results from the sensors and the joint field campaign.

Soil moisture plays an important role for the understanding of hydrological processes due to its influence on water and energy fluxes between the soil surface and the atmosphere. Knowledge of soil water dynamics is especially critical in water-scarce areas. In agroforestry systems, for instance, excessive competition for water between the trees and crops might outweigh the benefits of the system, thus preventing a successful implementation.Several techniques exist for measuring soil moisture and commercial devices vary widely in cost, reliability and efficiency. An alternative approach could be to estimate soil moisture dynamics from soil thermal dependencies. Similar approaches are already being used in remote sensing, as soil moisture influences the soil thermal properties and thus the surface energy balance and soil heat transfer. However, few studies have tested the feasibility of estimating in-situ soil moisture dynamics from soil temperature dynamics within a soil profile. Temperature sensors are cheaper, smaller and technically robust and could thus provide an interesting alternative to available commercial soil moisture sensors.In this study, we quantify the effect of soil moisture on phase shift and amplitude attenuation of soil temperature to estimate soil moisture content. We investigate these relationships from two different angles. Firstly, we use virtual measurements in coupled model simulations of soil water and soil heat dynamics to infer the general feasibility and precision of the method in an idealized error-free world. A sensitivity analysis can give insights on how the parametrization of the thermal diffusivity affects the precision and feasibility. Secondly, we compare findings from these simulations to results from analyzing time series of both soil moisture and soil temperature measured in an agroforestry field site in South Africa. A tentative analysis of these time series reveals that the amplitude attenuation and phase shift in the daily temperature signal is clearly sensitivity to changes in soil moisture. Finally, we aim to setup a coupled model for the study site based on the available soil hydraulic and textural data and compare simulated with observed phase shifts and attenuations at different depths.

Twoin situoptical sensors, a single-excitation fluorescence-based sensor (fDOM) mounted on a multi-parameter EXO2 sonde (YSI), and a stand-alone, multispectral absorbance-based instrument (spectro::lyser, scan Messtechnik GmbH), were evaluated for their capability to (i) estimate river dissolved organic carbon (DOC) concentrations and (ii) provide oversight of drinking water production. The sensors were deployed between March and November 2017 in the river Fyris, which drains a mixed forested and agricultural 2003 km(2)catchment and serves as a drinking water source by managed aquifer recharge. Grab samples were collected every 2 to 3 weeks and compared with logged sensor data collected at 15 minute intervals. The fDOM probe signal was used to estimate DOC concentrations in the range of 10.4 to 24.4 mg L(-1)using linear regression (R-2= 0.71, RMSE = 2.5 mg L-1), after correction for temperature, turbidity and inner-filter effects. Temporal changes in DOC character associated with the mixed land use landscape, as indicated by optical indices, reduced this sensor accuracy for estimating DOC concentration. Nevertheless, humic substance concentrations, the fraction of DOC that is preferentially removed during artificial infiltration, were well captured. The spectrolyser signal was used to establish a 2-component partial least square model that captured DOC fluctuations from 10.2 to 29.4 mg L-1(R-2= 0.92; RMSE = 1.3 mg L-1). This multiple-wavelength model (220 to 720 nm) effectively handled the changes in DOC composition while accurately estimating DOC concentrations. This study explores the advantages and limitations of optical sensors for their use in managed aquifer recharge and drinking water production in relation to DOC levels.

Agroforestry is often discussed as a strategy that can be used both for the adaptation to and the mitigation of climate change effects. The climate of southern Africa is predicted to be severely affected by such changes. With agriculture noted as the continent’s largest economic sector, issues such as food security and land degradation are in the forefront. In the light of such concerns we review the current literature to investigate if agroforestry systems (AFS) are a suitable response to the challenges besetting traditional agricultural caused by a changing climate. The benefits bestowed by AFS are multiple, offering ecosystem services, influence over crop production and positive impacts on rural livelihoods through provisioning and income generation. Nevertheless, knowledge gaps remain. We identify outstanding questions requiring further investigation such as the interplay between trees and crops and their combination, with a discussion of potential benefits. Furthermore, we identify deficiencies in the institutional and policy frameworks that underlie the adoption and stimulus of AFS in the southern African region. We uphold the concept that AFS remains an appropriate and sustainable response for an increased resilience against a changing climate in southern Africa for the benefit of livelihoods and multiple environmental values.

Expected changes in water quality due to a changing climate associated with variabilities in rainfall, runoff and diffusive spreading of pollution pose an emerging risk for the drinking water supply. Extensive monitoring of water quality parameters is of importance and the possibility of sudden extreme events brings up the need of high-frequency measurements. Dissolved organic carbon (DOC) has frequently been used as indicator for quality due to its capability of transporting contaminants and heavy metals, and its negative influence on the water treatment process. Hence, two sensors equipped with optical probes measuring fluorescence at an excitation of 365 nm and emission at 480 nm (EXO) and absorbance from 220 to 720 nm (s::can) were deployed in the Fyris river in Uppsala, central Sweden to approximate DOC concentrations. This is possible as the wavelength at which absorbance and fluorescence occur are unique for each molecule. Furthermore, the EXO carried a temperature and a turbidity probe. Measurements were conducted every 15 minutes for a period of about four months. Supplementary experiments improved the development of a calibration method for compensating temperature and turbidity effects on fluorescence measurements, as well as the generation of an algorithm to estimate DOC and turbidity from spectral absorbance data. Very stable weather and discharge conditions allowed calibration only for a small data range in all variables. However, this condition favored the chance of observing a change in DOC character throughout the measuring period, which was confirmed by performing monthly laboratory analysis of DOC, TOC (total OC), fluorescence and absorbance at four different stations along the river. It became apparent that both sensors do not capture this change in composition very well, whereupon the s::can performs slightly better. The rather simple algorithm delivers reasonable estimates of DOC and turbidity, which can be improved when data with a greater variability are available. Inter-calibration of the sensors demonstrated systematic errors between the two and laboratory measurements. The dry-spell in Uppsala revealed how the parameters from different stations become more alike as the groundwater fraction of the stream water increases. The change in character depicted by the specific absorbance in the ultraviolet range suggests that during water treatment greater effort in the flocculation process will be needed. Generellt förväntar sig många i Sverige att kranvattnet är drickbart. Många av oss set det som en självklarhet att kunna dricka friskt kranvatten. Därför, är det extra viktigt att upprätthålla denna goda kvalité. Kostsamma laborationsmätningar görs regelbundet på enstaka vattenprover för att kontrollera vattenkvalitén. Mätningarna som görs är mycket pålitliga och noggranna, men problemet med övervakningen av vattenkvalitén är att förändringar sker väldigt fort. Exempelvis, skapar en kraftigt men kort regnskur en våg av partiklar, vilket kan ger problem vid reningen av vatten. Det är nästintill omöjligt, både tidsmässigt och pengamässigt, att gå ut i fält, samla extra vattenprover varje gång det regnar och samtidigt jobba och ta vanliga prover. Tänk om du, istället för att konstant vara fysiskt närvarande, hade ett par extra händer och ögon på plats som kan ta mätningar åt dig, på olika platser exakt samtidigt. Dessa extra händer existerar i form av sensorer som placeras direkt i vattnet och övervakar vattenkvalitén. Det här projektet fokuserar på att jämföra förmågan att mäta indikatorer för vattenkvalité hos två sensorer. Varför indikatorer? Kvalitén på vatten är ett komplext system som är beroende på många faktorer, som exempelvis temperatur, pH, olika partiklar och föroreningar som flyter i vattnet. Sensorerna kan inte mäta allt, de fokuserar istället på några få parametrar som är kända för att ge en indikation om hur bra vattenkvalitén är i allmänhet. En indikator som ofta används, bland annat för dess förmåga att transportera föroreningar, är löst organiskt kol (DOC; t. ex. nedbrytningsprodukter från växter). Idag finns det ingen enkel sensor för att mäta DOC. Men DOC skapar färg i vatten och har därför optiska egenskaper. Optiska sensorer (ljusstråle) mäter vattenfärg och man kan använda de för att bestämma koncentration av DOC. Särskilt enkelt är det inte att bara placera sensorn för fluorescensmätningar i vattnet. Ljusstrålarna är lättpåverkade av såväl partiklar som flyter i vattnet och av vattentemperaturen, vilket gör att korrigeringar för dessa behöver göras. Det har visat sig att temperaturen har en betydande effekt på mätningarna. Korrigeringar av optiska mätningar med två olika sensorer behövde därför korrigeras för att kunna jämföras mot laborationsmätningarna. Metoderna som används i den här studien behandlar utvärdering av DOC-halter. Under provperioden var väderförhållandena mycket stabila. Eftersom ingen hänsyn har tagits till extrema väderförhållanden (ex. storm) kan ingen slutsats dras angående pålitligheten hos resultaten under extrema förhållanden. En annan viktig felkälla i resultaten identifierades. När sammansättningen av kolet ändras, så kan mätresultatet vara missvisande. Framtida analyser måste ta hänsyn till detta.