Volume 67 (2019): Issue 3 (September 2019)

Increasing our understanding of the main processes acting in small Mediterranean catchments is essential to planning effective soil and water conservation practices in semi-arid areas. A monitoring program of a Sicilian catchment started in 1996 and ended in 2006. The factors driving the hydrological response for 170 events with runoff generation and 46 with sediment production were specified. The catchment response varied greatly over the year. Rainfall intensity was a poor driver of runoff generation, whereas both the simulations made with the Thornthwaite-Mather water balance model and hydrograph recession analyses, pointed to the chief importance of wet antecedent conditions and soil saturation processes in runoff generation. The influence of rainfall spatial variability was also examined. SSC-Q relationships, classified by following their shapes for all sediment production events, suggested that the principal role of small poorly vegetated hillslope patches was as sediment sources and confirmed the complexity of the hydrological response in this small Mediterranean catchment.

Evapotranspiration is often estimated by numerical simulation. However, to produce accurate simulations, these models usually require on-site measurements for parameterization or calibration. We have to make sure that the model realistically reproduces both, the temporal patterns of soil moisture and evapotranspiration. In this study, we combine three sources of information: (i) measurements of sap velocities; (ii) soil moisture; and (iii) expert knowledge on local runoff generation and water balance to define constraints for a “behavioral” forest stand water balance model. Aiming for a behavioral model, we adjusted soil moisture at saturation, bulk resistance parameters and the parameters of the water retention curve (WRC). We found that the shape of the WRC influences substantially the behavior of the simulation model. Here, only one model realization could be referred to as “behavioral”. All other realizations failed for a least one of our evaluation criteria: Not only transpiration and soil moisture are simulated consistently with our observations, but also total water balance and runoff generation processes. The introduction of a multi-criteria evaluation scheme for the detection of unrealistic outputs made it possible to identify a well performing parameter set. Our findings indicate that measurement of different fluxes and state variables instead of just one and expert knowledge concerning runoff generation facilitate the parameterization of a hydrological model.

Mixed evergreen-deciduous broadleaved forest is the transitional type of evergreen broadleaved forest and deciduous broadleaved forest, and plays a unique eco-hydrologic role in terrestrial ecosystem. We investigated the spatio-temporal patterns of throughfall volume of the forest type in Shennongjia, central China. The results indicated that throughfall represented 84.8% of gross rainfall in the forest. The mean CV (coefficient of variation) of throughfall was 27.27%. Inter-event variability in stand-scale throughfall generation can be substantially altered due to changes in rainfall characteristics, throughfall CV decreased with increasing rainfall amount and intensity, and reached a quasi-constant level when rainfall amount reached 25 mm or rainfall intensity reached 2 mm h⁻¹. During the leafed period, the spatial pattern of throughfall was highly temporal stable, which may result in spatial heterogeneity of soil moisture.

Application of compost is known to improve the hydraulic characteristics of soils. The objective of this study was to examine the seasonal and short-term effects of solid waste compost amendments on selected hydrophysical properties of soil during dry and rainy seasons and to explore any negative impacts of municipal solid waste compost (MSWC) amendments on soil hydrophysical environment concerning Agriculture in low-country wet zone, Sri Lanka. Eight (T1–T8) MSWC and two (T9, T10) agricultural-based waste compost (AWC) samples were separately applied in the field in triplicates at 10 and 20 Mg ha⁻¹ rates, with a control (T0). Field measurements (initial infiltration rate, I_i; steady state infiltration rate, I_SS; unsaturated hydraulic conductivity, k; sorptivity, S_W) were conducted and samples were collected (0–15 cm depth) for laboratory experiments (water entry value, h_we; potential water repellency: measured with water drop penetration time, WDPT) before starting (Measurement I) and in the middle of (Measurement II) the seasonal rainfall (respectively 5 and 10 weeks after the application of compost). The difference in the soil organic matter (SOM) content was not significant between the dry and rainy periods. All the soils were almost non-repellent (WDPT = <1–5 s). The h_we of all the samples were negative. In the Measurement I, the I_i of the T0 was about 40 cm h⁻¹, while most treatments show comparatively lower values. The I_SS, S_W, and k of compost amended samples were either statistically similar, or showed significantly lower values compared with T0. It was clear that all the surface hydraulic properties examined in situ (I_i, I_SS, S_W) were higher in the Measurement I (before rainfall) than those observed in the Measurement II (after rainfall). Water potential differences in soils might have affected the surface hydrological properties such as S_W. However, water potential differences would not be the reason for weakened I_SS and k in the Measurement II. Disruption of aggregates, and other subsequent processes that would take place on the soil surface as well as in the soil matrix, such as particle rearrangements, clogging of pores, might be the reason for the weakened I_SS and k in the Measurement II. Considering the overall results of the present study, compost amendments seemed not to improve or accelerate but tend to suppress hydraulic properties of soil. No significant difference was observed between MSWC and AWC considering their effects on soil hydraulic properties. Application of composts can be considered helpful to slower the rapid leaching by decreasing the water movements into and within the soil.

In the present study, experiments were conducted in a large-scale flume to investigate the issue of local scour around side-by-side bridge piers under both ice-covered and open flow conditions. Three non-uniform sediments were used in this experimental study. Analysis of armour layer in the scour holes around bridge piers was performed to inspect the grain size distribution curves and to study the impact of armour layer on scour depth. Assessments of grain size of deposition ridges at the downstream side of bridge piers have been conducted. Based on data collected in 108 experiments, the independent variables associated with maximum scour depth were assessed. Results indicate that the densi-metric Froude number was the most influential parameter on the maximum scour depth. With the increase in grain size of the armour layer, ice cover roughness and the densimetric Froude number, the maximum scour depth around bridge piers increases correspondingly. Equations have been developed to determine the maximum scour depth around bridge piers under both open flow and ice covered conditions.

A 2D hydrodynamic (labeled as CAR) model has been proposed in a rectangular Cartesian coordinate system with two axes within the horizontal plane and one axis along the vertical direction (global coordinates), considering the effects of bed slope on both pressure distribution and bed shear stresses. The CAR model satisfactorily reproduces the analytical solutions of dam-break flow over a steep slope, while the traditional Saint-Venant Equations (labeled as SVE) significantly overestimate the flow velocity. For flood events with long duration and large mean slope, the CAR and the SVE models present distinguishable discrepancies. Therefore, the proposed CAR model is recommended for applications to real floods for its facility of extending from 1D to 2D version and ability to model shallow-water flows on steep slopes.

Subtropical regions have clay-rich, weathered soils, and long dry periods followed by intense rainfall that produces large fluctuations in soil water content (SWC) and hydrological behavior. This complicates predictions of spatio-temporal dynamics, as datasets are typically collected at too coarse resolution and observations often represent a duration that is too short to capture temporal stability. The aim of the present study was to gain further insights into the role of temporal sampling scale on the observed temporal stability features of SWC order to aid the design of optimal SWC sampling strategies. This focused on both sampling frequency and total monitoring duration, as previous analyses have not considered both of these sampling aspects simultaneously. We collected relatively high resolution data of SWC (fortnightly over 3.5 years) for various soil depths and under contrasting crops (peanuts and citrus) at the red soil region of southeast China. The dataset was split into a three-year training period and a six-month evaluation period. Altogether 13 sampling frequencies (intervals ranging from 15 to 240 days) and eight monitoring duration periods (between three and 36 months) were derived from the training period to identify temporal stability features and the most time stable location (MTSL). The prediction accuracies of these MTSLs were tested using the independent evaluation data. Results showed that vegetation type did affect the spatio-temporal patterns of SWC, whereby the citrus site exhibited a stronger temporal variation and weaker temporal stability than the peanut site. However, the effects of both sampling frequency and observation duration were more pronounced, irrespective of the role of vegetation type or soil depth. With increasing sampling interval or decreasing monitoring duration, temporal stability of SWC was generally overestimated; by less than 10% when sampling interval increased from every 15 to 240 days and by up to 40% with duration decreasing from 36 to 3 months. Our results suggest that sampling strategies and trade-offs between sampling interval and duration should focus on capturing the main variability in hydro-climatological conditions. For subtropical conditions, we found that sampling once every 45 days over 24 months to be the minimum sampling strategy to ensure errors in SWC temporal stability of less than 10%.

Tree transpiration plays a determining role in the water balance of forest stands and in seepage water yields from forested catchments, especially in arid and semiarid regions where climatic conditions are dry with severe water shortage, forestry development is limited by water availability. To clarify the response of water use to climatic conditions, sap flow was monitored by heat pulse velocity method from May to September, 2014, in a 40–year–old Pinus tabulaeformis Carr. plantation forest stands in the semiarid Loess Plateau region of Northwest China. We extrapolated the measurements of water use by individual plants to determine the area–averaged transpiration of the woodlands. The method used for the extrapolation assumes that the transpiration of a tree was proportional to its sapwood area. Stand transpiration was mainly controlled by photosynthetically active radiation and vapor pressure deficit, whereas soil moisture had more influence on monthly change in stand transpiration. The mean sap flow rates for individual P. tabulaeformis trees ranged from 9 to 54 L d⁻¹. During the study period, the mean daily stand transpiration was 1.9 mm day⁻¹ (maximum 2.9 and minimum 0.8 mm day⁻¹) and total stand transpiration from May to September was 294.1 mm, representing 76% of the incoming precipitation over this period. Similar results were found when comparing transpiration estimated with sap flow measurements to the Penman–Monteith method (relative error: 16%), indicating that the scaling procedure can be used to provide reliable estimates of stand transpiration. These results suggested that P. tabulaeformis is highly effective at utilizing scarce water resources in semiarid environments.

Longwave radiation, as part of the radiation balance, is one of the factors needed to estimate potential evapotranspiration (PET). Since the longwave radiation balance is rarely measured, many computational methods have been designed. In this study, we report on the difference between the observed longwave radiation balance and modelling results obtained using the two main procedures outlined in FAO24 (relying on the measured sunshine duration) and FAO56 (based on the measured solar radiation) manuals. The performance of these equations was evaluated in the April–October period over eight years at the Liz experimental catchment and grass surface in the Bohemian Forest (Czech Republic). The coefficients of both methods, which describe the influence of cloudiness factor and atmospheric emissivity of the air, were calibrated. The Penman-Monteith method was used to calculate the PET. The use of default coefficient values gave errors of 40–100 mm (FAO56) and 0–20 mm (FAO24) for the seasonal PET estimates (the PET was usually overestimated). Parameter calibration decreased the FAO56 error to less than 20 mm per season (FAO24 remained unaffected by the calibration). The FAO56 approach with calibrated coefficients proved to be more suitable for estimation of the longwave radiation balance.

Employing evapotranspiration models is a widely used method to estimate reference evapotranspiration (ET_REF) based on weather data. Evaluating such models considering site-specific boundary conditions is recommended to interpret ET_REF-calculations in a realistic and substantiated manner. Therefore, we evaluated the ASCE standardized ET_REF-equations at a subhumid site in northeastern Austria. We calculated ETREF-values for hourly and daily time steps, whereof the former were processed to sum-of-hourly values. The obtained data were compared to each other and to ET-values measured by a weighing lysimeter under reference conditions. The resulting datasets covered daily data of the years 2004 to 2011.

Sum-of-hourly values correlated well (r² = 0.978) with daily values, but an RMSE of 0.27 mm specified the differences between the calculation procedures. Comparing the calculations to lysimeter measurements revealed overestimation of small ET_REF-values and underestimation of large values. The sum-of-hourly values outperformed the daily values, as r² of the former was slightly larger and RMSE was slightly smaller. Hence, sum-of-hourly computations delivered the best estimation of ET_REF for a single day. Seasonal effects were obvious, with computations and measurements being closest to each other in the summer months.