Browsing by Subject "Soil moisture"

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Measuring and modelling of soil water dynamics in two German landscapes
(2018) Poltoradnev, Maksim; Streck, Thilo
The soil water regime is focus of various disciplines including agricultural sciences, hydrology, weather forecast and climate modelling. As an inherent part of land surface exchange processes, the dynamics of soil water content (SWC) is simulated in distributed hydrological models and land surface models (LSM). The accuracy of the simulated SWC directly influences the simulation outcome and its performance. Biases in modelled temporal SWC dynamics and its spatial distribution lead to errors in evapotranspiration, runoff, cloud and precipitation simulations. The main objective of my thesis was to study the factors that control the SWC dynamics and its spatial variability. Long-term measurements from the soil moisture networks Kraichgau (KR) and Swabian Alb (SA) provided the data basis of this study. SWC was sensed based on the Time Domain Transmission (TDT) technique. In each region, 21 measuring locations were distributed across three spatial domains: an inner domain 3 km × 3 km (5 stations), a middle 9 km × 9 km (8 stations), and an outer domain 27 km × 27 km (8 stations). The sizes of the three domains correspond with typical grid sizes of coupled atmosphere-LSM models. All stations were mounted on cropped agricultural sites. Each station was equipped with a TDT sensor, installed 15 cm deep into the soil, a rain gauge and a remote transfer unit. After adjusting the sensor networks, an in-situ field calibration was performed to derive pedotransfer and site-specific calibrations for TDT soil moisture sensors. The chemical and physical analysis of soil samples collected at each station revealed that soil bulk density influences in both regions the TDT readings. Moreover, the pedotransfer calibrations included electrical conductivity in KR and silt fraction and organic nitrogen content on SA. These variables are relatively easy to measure. Accordingly, the pedotransfer calibrations derived in this study are a quick possibility to calibrate TDT sensors in areas with similar soil properties as in KR and SA. Nevertheless, the site-specific calibrations performed the best and were therefore used for further data analysis. In the second study, a three-year record of SWC and rainfall was evaluated. The response of the regional mean (theta) of SWC to a rain event was influenced by the seasonal water balance (SWB). In KR, the relation was more pronounced for positive SWB and less for neutral and negative SWB. On SA, where SWB was highly positive in all three years, the response of theta to rainfall was always strong. At the seasonal scale, the relationship between the spatial standard deviation of SWC (sigma) and theta was investigated through sigma-theta phase-space diagrams. The results show that with decreasing SWC sigma-theta data pairs are approaching sigma at the permanent wilting point (sigma-thetawp). With increasing SWC, in contrast, sigma-theta data pairs are moving towards sigma at saturation (sigma-thetas). These two points were termed anchor points. The sigma-theta relationships formed combinations of concave and convex hyperbolas reflecting the variability of soil texture and depending on sigma in relation to the anchor points. At the event scale, hysteresis in the sigma-theta was observed. Most sigma-theta clockwise hysteresis cases occurred at an intermediate and intermediate/wet state of SWC. Among the factors that trigger the initiation of a sigma-theta hysteretic loop, the present study revealed the following: rainstorms with spatially highly variable intensities (threshold rainfall intensity of 1.1 ± 0.6 mm and 2.9 ± 2.8 mm for KR and SA, respectively), preferential flow and, possibly, hysteresis in soil water retention curves. Based on these results, the following hypothesis was formulated: sigma-theta phase space diagrams are useful to test whether hydrological models or land surface models (LSMs) capture the realistic range of spatial soil water variability. The concept was tested with the Noah-MP LSM. Observations obtained from KR and SA soil moisture networks over a three-year period from 2010 to 2012 were used to build up the sigma-theta phase-space. The study included two different setups used to compute the hydraulic conductivity and the diffusivity: 1) the default setting: the Clapp and Hornberger approach, and 2) the van Genuchten-Mualem functions. The default model parameterization was stepwise substituted with site-specific rainfall, soil texture, leaf area index (LAI) and green vegetation fraction (GVF) data. The atmospheric forcing was obtained from eddy covariance stations located in the regions. Although the model matched observed temporal theta dynamics fairly well for the loess soils of KR, it performed poorly in the case of the shallow, clayey and stony soils of SA. The best match was achieved with the van Genuchten-Mualem functions and site-specific rainfall, soil texture, GVF and LAI. Nevertheless, the Noah-MP LSM failed to represent the spatial variability of SWC. In most cases, the simulated sigma-theta data points were located below the bottom edge of the envelope, which indicates that the model smooths spatial variability of soil moisture. This smoothing can be mainly attributed to missing topography and terrain information, inadequate representation of the spatial variability of soil texture and hydraulic parameters, and the model assumption of a uniform root distribution.
Soil moisture dynamics in integrated crop - livestock - forestry systems in the Cerrado Biome in Central - West Brazil
(2021) Glatzle, Sarah; Asch, Folkard
The Cerrado biome in Brazil covers about 200 million ha and is a global biodiversity hotspot. Over the last decades, the Cerrado biome underwent and is still undergoing an excessive expansion in agriculture. Deforestation and replacement of the natural Savannah vegetation by cropland and pasture contributes to serious environmental problems, including soil degradation and altered water cycles. The integrated crop-livestock-forestry (ICLF) system is currently promoted as a measure for sustainable intensification. It improves the use of cultivated areas, recovers previously degraded land, and could be a strategy for adapting agriculture to climate change. Despite being considered a key indicator of how integrated systems affect ecological processes, soil moisture (SM) dynamics in literature have not been consistently analyzed, and continuous observation of seasonal SM dynamics are mostly unaddressed. Since SM of complex ecosystems is influenced by numerous factors, several additional parameters need to be considered to create a comprehensive understanding of the interlinked processes, such as radiation, rainfall, and biomass. The objective of this cumulative PhD thesis was to investigate SM dynamics and aboveground grass biomass under different land use systems in the Cerrado biome of Central West Brazil. In the first study, photosynthetically active radiation (PAR) received at grass canopy level, SM, AGBM between the tree rows, and seasons in a mature ICLF system were investigated. Across the seasons, a distinct gradient was observed with SM being lower close to the tree rows than in the space between them. During winter, SM in the topsoil decreased to critical values, and dropped to the permanent wilting point next to the tree rows. During spring and summer, incident PAR was lower close to the trees than at the center point, while during autumn and winter, when PAR is generally lower, it was more evenly distributed between the tree rows. Aboveground grass biomass (AGBM) showed a distinct distribution within the ICLF system with maximum values in the center and about 50% of the biomass close to the tree rows. The results suggest that, restrictions in AGBM accumulation shifted among seasons between water limitations in winter and light limitations during summer. In the second study, the seasonal and spatial variability of SM of Cerrado soils under four different land use systems was investigated under consideration of soil physical characteristics and grass biomass. In rainy and dry season, SM in the upper 100 cm of the soil was highest in the integrated crop-livestock (ICL) system, followed by the continuous pasture (COP), and lowest in the land use systems including trees, ICLF and Cerrado. Whereas in COP and in ICL, water was mainly taken up from the upper 30 cm, in ICLF, the strongest soil moisture depletion was observed between a soil depth of 40 and 100 cm. Although in the Cerrado SM in the topsoil was lower than in the other land use types, water was conserved below 60 cm depth. Both integrated systems improved soil properties, such as bulk density and soil organic carbon compared to COP, and increased biomass productivity was observed, demonstrating the benefits of the integrated systems over the traditional grazing system. The results suggest that ICLF systems show increased evapotranspiration compared to conventional pasture and other integrated systems without trees. In the third study, the effects of the presence of eucalyptus trees on the seasonal pasture and animal performance in ICLF systems 8 years after establishment were investigated. Forage morphology, production, and nutritive value plus performance of Nellore heifers in two ICLF systems with varying in trees density, were evaluated and compared with a grass-only pasture. In both ICLF systems, the forage nutritive values were improved compared with a grass-only pasture. Nevertheless, grass biomass and accumulation rate were higher in the grass-only pasture. By the 8th year, the ICLF systems were unable to support both forage and animal production equivalent to a grass-only pasture, due to the high impact of the Eucalyptus trees on radiation received at the grass canopy and on soil moisture. Improved soil characteristics and forage nutritive values compared to grass-only pastures, and the potential restoration of natural ecosystem functions regarding water recycling into the atmosphere, demonstrated the benefits of ICLF systems and highlight their potential to contribute to sustainable agricultural intensification. However, high water consumption by trees poses a risk to grass productivity during the dry season and thus, the system may consequently not be used for grazing all year round. Therefore, research on management options mitigating the impact of drought on grass productivity is needed. As the impact of the trees on the system is highly dependent on their age, these studies should consider the entire life cycle of the system.