Application of an optical disdrometer to characterize simulated rainfall and measure drop-size distribution

ABSTRACT

Knowledge of rainfall characteristics such as drop-size distribution is essential for the development of erosion-mitigation strategies and models. This research used an optical disdrometer to elucidate the relationships between raindrop-size distribution, median volume drop diameter (D₅₀), kinetic energy and radar reflectivity (dBz) of simulated rainfall of different intensities. The D₅₀ values were higher for the simulated rain than for natural rain at almost all rainfall intensities, perhaps due to variations in rainfall types and the turbulence in natural rain that breaks up large drops. The kinetic energy ranged from 26.67 to 5955.51 J m^?2 h ^?1, while the median volume drop diameter (D₅₀) was in the range 1.94–7.25 mm, for intensities between 1.5 and 202.6 mm h^?1. The relationship between radar reflectivity (Z) and the intensity (R) of the simulated rain was best described by a power law function (Z = aR^b), with a and b coefficients in the ranges 162–706 and 0.94–2.46, respectively, throughout the range of rainfall intensities (1.5–202.6 mm h^?1).     

Dynamic characteristics of soil-foundation interaction system detected from forced vibration test and earthquake observation

This paper presents some results on the following subjects obtained from in-situ forced vibration tests and earthquake observations. (1) The characteristics of the radiation damping of soil-foundation interaction systems vs. non-dimensional frequency a₀ (=ωr/V_s) were experimentally estimated by the equivalent damping ratios h_H ( = K′_H/2K_H) and h_R ( = K′_R/2K_R), which were defined by complex stiffnesses 1K_H (= K_H + iK_H) and 1K_R (= K_R + iK′_R) of soil. The results for h_H and h_R of base rock were compared with those of soft soil. (2) A comparative study of experimental and theoretical results was made. The theoretical results were obtained from elastic half-space theory. (3) A semi-empirical equation to estimate the equivalent S-wave velocity for the elastic half-space model is proposed here, considering the effects of layered media. (4) Various comparisons of the results of 1 K_H, 1 K_R, h_H and h_R of forced vibration tests and earthquake observations were made.     

Simulating Z–LWC Relations in Natural Fogs with Radiative Transfer Calculations for Future Application to a Cloud Radar Profiler

The vertical distribution of liquid water content (LWC) in natural fog and low stratus is a crucial variable in many applications, e.g. the development of satellite based retrievals of ground fog. Unfortunately, there is very little data concerning fog LWC-profiles, mainly due to the lack of suitable operational instrumentation. A novel ground-based 94?GHz FMCW cloud radar could fill this gap if radar reflectivity Z could be converted to LWC by using appropriate Z–LWC relations. However, this relation strongly depends on drop size distribution (DSD) and is hardly known for natural fog types. In this sensitivity study, the influence of the DSD on the Z–LWC relation in different types and life cycle stages of natural fogs is analyzed using a radiative transfer code (RTC) and published fog drop size distributions. It could be shown that there is a direct but nonlinear relationship between LWC and radar reflectivity. The proportionality factor of the Z–LWC equation in particular reveals specific ranges for the different life cycle stages. If a proper classification of fog life cycle in the field is possible, the results could be used to properly convert Z to LWC.     

Differences in throughfall drop size distributions in the presence and absence of foliage

ABSTRACT

Throughfall drop size distributions (DSDs) are important for plant–soil interactions. This is the first known study to quantify differences in throughfall DSDs with the presence and absence of foliage. Employing a disdrometer, three parameters solely representing throughfall drip were measured and calculated: maximum drop diameter (D_MAX), median volume diameter of drops (D₅₀D_R) and relative volume percentage of drops (pD_R). Beneath Liriodendron tulipifera L. in Maryland (USA), D_MAX, D₅₀D_R and pD_R were substantially larger when the canopy was unfoliated. In fact, the presence or absence of foliage was one of the primary factors affecting all three throughfall DSDs along with air temperature, according to the boosted regression tree analysis. Experimental results were attributed to differing physical properties of intercepted water between foliated and unfoliated periods and differential water behavior on leaves and bark. Future work should examine the effects of concentrated drip points on the development of throughfall-induced hot spots.

Editor M.C. Acreman; Associate editor F. Hattermann     

Scaling characteristics of SEGMA magnetic field data around the Mw 6.3 Aquila earthquake

We apply detrended fluctuation analysis (DFA) on fluxgate and search-coil data in ULF range (scales 10–90 s or 0.1–0.011 Hz) for the months January–April 2009 available from the South European GeoMagnetic Array stations: Castello Tesino (CST), Ranchio (RNC), and L’Aquila (AQU) in Italy; Nagycenk (NCK) in Hungary; and Panagyuriste (PAG) in Bulgaria. DFA is a data processing method that allows for the detection of scaling behaviors in observational time series even in the presence of non-stationarities. The H and Z magnetic field components at night hours (00-03 UT, 01–04 LT) and their variations at the stations CST, AQU, NCK, and PAG have been examined and their scaling characteristics are analyzed depending on geomagnetic and local conditions. As expected, the scaling exponents are found to increase when the K _p index increases, indicating a good correlation with geomagnetic activity. The scaling exponent reveals also local changes (at L’Aquila), which include an increase for the Z (vertical) component, followed by a considerable decrease for the X (horizontal) component in the midst of February 2009. Attempts are made to explain this unique feature with artificial and/or natural sources including the enhanced earthquake activity in the months January–April 2009 at the L’Aquila district.     

Lg amplitude ratios H/Z and station terms in the six eastern provinces of China

Using the data set of about 1 Hz Lg amplitudes from 80 stations in the six eastern provinces of China, the ratios of horizontal to vertical amplitude of Lg waves,H/Z are determined. The mean lg(H/Z)-value is equal to 0.13. For the station bases of soft deposit,granite and sedimentary rock, lg(H/Z)-values average 0.55, 0.04 and 0.10, respectively. For the three kinds of station bases, the station terms of horizontal amplitude,C _h average 0.65, −0.09 and 0.00; that of vertical amplitude,C _z average 0.32, −0.04 and 0.00, respectively. There are the relations betweenC _h ,C _z and lg(H/Z):C _h =0.01+2.10C _z andC _h =−0.15+1.46 lg(H/Z). The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 281–286, 1991. Project sponsored by the Chinese Joint Seismological Science Foundation.     

Using a transient infiltrometric technique for intensively sampling field‐saturated hydraulic conductivity of a clay soil in two runoff plots

The point measurement of soil properties allows to explain and simulate plot scale hydrological processes. An intensive sampling was carried out at the surface of an unsaturated clay soil to measure, on two adjacent plots of 4 × 11 m² and two different dates (May 2007 and February–March 2008), dry soil bulk density, ρ_b, and antecedent soil water content, θ_i, at 88 points. Field‐saturated soil hydraulic conductivity, K_fs, was also measured at 176 points by the transient Simplified Falling Head technique to determine the soil water permeability characteristics at the beginning of a possible rainfall event yielding measurable runoff. The ρ_b values did not differ significantly between the two dates, but wetter soil conditions (by 31%) and lower conductivities (1.95 times) were detected on the second date as compared with the first one. Significantly higher (by a factor of 1.8) K_fs values were obtained with the 0.30‐m‐diameter ring compared with the 0.15‐m‐diameter ring. A high K_fs (> 100 mm h^?1) was generally obtained for low θ_i values (< 0.3 m³m^?3), whereas a high θ_i yielded an increased percentage of low K_fs data (1–100 mm h^?1). The median of K_fs for each plot/sampling date combination was not lower than 600 mm h^?1, and rainfall intensities rarely exceeded 100 mm h^?1 at the site. The occurrence of runoff at the base of the plot needs a substantial reduction of the surface soil permeability characteristics during the event, probably promoted by a higher water content than the one of this investigation (saturation degree = 0.44–0.62) and some soil compaction due to rainfall impact. An intensive soil sampling reduces the risk of an erroneous interpretation of hydrological processes. In an unstable clay soil, changes in K_fs during the event seem to have a noticeable effect on runoff generation, and they should be considered for modeling hydrological processes. Copyright © 2012 John Wiley & Sons, Ltd.     

Factors affecting the measurement of the vertical hydraulic conductivity of a streambed sediment using standpipe tests

The vertical hydraulic conductivity (K_v) of a stream or lake sediment is often determined in the field using standpipe tests. Calculation of K_v is based on the assumption that the hydraulic head in the pipe is equal to that of the stream or lake stage. In this work, a modified equation for K_v is developed for the standpipe test which is applicable when this assumption is not valid. The equation involves not only the hydraulic head at different times but also the difference in the hydraulic head (a) between the groundwater level and river stage. The effects of certain factors on K_v, such as the ratio of the hydraulic head at different times (h₁/h₂), the difference a, and the initial water table height (h₀), are also discussed. The results show that when h₁/h₂ is constant, the relative error (E_r) in K_v increases with the ratio a/h₂. Furthermore, if a/h₂ < 0.05, then for any value of h₁/h₂, E_r is less than 5% using the modified equation. Also, if a/h₂ is large, hydraulic head readings with larger h₁/h₂ ratios must be used to avoid large E_r values. The results of a field test also indicate that the error in K_v decreases as the value of h₀ increases. Copyright © 2013 John Wiley & Sons, Ltd.     

Upward infiltration–evaporation method to estimate soil hydraulic properties

Determination of saturated hydraulic conductivity, K_s, and the van Genuchten water retention curve θ(h) parameters is crucial in evaluating unsaturated soil water flow. The aim of this work is to present a method to estimate K_s, α and n from numerical analysis of an upward infiltration process at saturation (Cap0), with (Cap0 + h) and without (Cap0) an overpressure step (h) at the end of the wetting phase, followed by an evaporation process (Evap). The HYDRUS model as well as a brute-force search method were used for theoretical loam soil parameter estimation. The uniqueness and the accuracy of solutions from the response surfaces, K_s–n, α–n and K_s–α, were evaluated for different scenarios. Numerical experiments showed that only the Cap0 + Evap and Cap0 + h + Evap scenarios were univocally able to estimate the hydraulic properties. The method gave reliable results in sand, loam and clay-loam soils.     

Persistence of road runoff generation in a logged catchment in Peninsular Malaysia

Measurements of saturated hydraulic conductivity (K_s) and diagnostic model simulations show that all types of logging road/trail in the 14·4 ha Bukit Tarek Experimental Catchment 3 (BTEC3) generate substantial Horton overland flow (HOF) during most storms, regardless of design and level of trafficking. Near‐surface K_s(0–0·05 m) on the main logging road, skid trails and newly constructed logging terraces was less than 1, 2 and 34 mm h^?1, respectively. Near‐surface K_s on an abandoned skid trail in an adjacent basin was higher (62 mm h^?1), owing to the development of a thin organic‐rich layer on the running surface over the past 40 years. Saturated hydraulic conductivity measured at 0·25 m below the surface of all roads was not different (all <6 mm h^?1) and corresponded to the K_s of the adjacent hillslope subsoil, as most roads were excavated into the regolith more than 0·5–1 m. After 40 years, only limited recovery in near‐surface K_s occurred on the abandoned skid trail. This road generated HOF after the storage capacity of the upper near‐surface layer was exceeded during events larger than about 20 mm. Thus, excavation into low‐K_s substrate had a greater influence on the persistence of surface runoff production than did surface compaction by machinery during construction and subsequent use during logging operations. Overland flow on BTEC3 roads was also augmented by the interception of shallow subsurface flow traveling along the soil–saprolite/bedrock interface and return flow emerging from the cutbank through shallow biogenic pipes. The most feasible strategy for reducing long‐term road‐related impacts in BTEC3 is limiting the depth of excavation and designing a more efficient road network, including minimizing the length and connectivity of roads and skid trails. Copyright © 2007 John Wiley & Sons, Ltd.     

Seasonal climate patterns and their influence on calibration of the Hargreaves-Samani equation

Abstract

Annual patterns in climate parameters were studied to evaluate how these influence the quality of reference evapotranspiration (ETo) estimates obtained from the Hargreaves-Samani (HS) equation, since the method only uses the measured temperature directly. The work evaluates how these patterns can be used to improve the HS ETo estimates. Ten-year moving averages from a set of California Irrigation Management Information System (CIMIS) stations were used to evaluate the relationships between solar radiation (R_s), temperature (T) and ETo. The results indicate that T treads behind solar radiation and its value peaks some 25 days later. Thus, the main irrigation season in the Mediterranean climate (1 May–30 September) can be divided into three phases: increasing R_s and T; decreasing R_s with increasing T; and decreasing R_s and T. Non-univocal annual cycles were observed between R_s and T, ETo and R_s, and ETo and T. These annual patterns result in important seasonal changes in the ratio between the HS and Penman-Monteith (FAO PM) ETo estimates. The changes are particularly important during the irrigation season, where the FAO PM initially calculates greater ETo values than the HS methodology, and from the end of May to early September, where the HS equation overestimates the ETo values (by 17 mm, or 3%). These patterns obtained from 2000–2009 data were used to calibrate and improve HS ETo estimates at new sites for the 2010–2011 period. Calibration based on the proposed seasonal region-wide FAO PM/HS ETo ratios improved both the bias (decreased from 0.40 to 0.36 mm d^-1) and r² (increased from 0.67 to 0.87) of the ETo estimates for the irrigation season. The proposed methodology can be easily applied to other regions, even when the existing weather stations are sparse.

Editor Z.W. Kundzewicz     

Developing an empirical model of canopy water flux describing the common response of transpiration to solar radiation and VPD across five contrasting woodlands and forests

A modified Jarvis–Stewart model of canopy transpiration (E_c) was tested over five ecosystems differing in climate, soil type and species composition. The aims of this study were to investigate the model's applicability over multiple ecosystems; to determine whether the number of model parameters could be reduced by assuming that site‐specific responses of E_c to solar radiation, vapour pressure deficit and soil moisture content vary little between sites; and to examine convergence of behaviour of canopy water‐use across multiple sites. This was accomplished by the following: (i) calibrating the model for each site to determine a set of site‐specific (SS) parameters, and (ii) calibrating the model for all sites simultaneously to determine a set of combined sites (CS) parameters. The performance of both models was compared with measured E_c data and a statistical benchmark using an artificial neural network (ANN). Both the CS and SS models performed well, explaining hourly and daily variation in E_c. The SS model produced slightly better model statistics [R² = 0.75–0.91; model efficiency (ME) = 0.53–0.81; root mean square error (RMSE) = 0.0015–0.0280 mm h^‐1] than the CS model (R² = 0.68–0.87; ME = 0.45–0.72; RMSE = 0.0023–0.0164 mm h^‐1). Both were highly comparable with the ANN (R² = 0.77–0.90; ME = 0.58–0.80; RMSE = 0.0007–0.0122 mm h^‐1). These results indicate that the response of canopy water‐use to abiotic drivers displayed significant convergence across sites, but the absolute magnitude of E_c was site specific. Period totals estimated with the modified Jarvis–Stewart model provided close approximations of observed totals, demonstrating the effectiveness of this model as a tool aiding water resource management. Analysis of the measured diel patterns of water use revealed significant nocturnal transpiration (9–18% of total water use by the canopy), but no Jarvis–Stewart formulations are able to capture this because of the dependence of water‐use on solar radiation, which is zero at night. Copyright © 2012 John Wiley & Sons, Ltd.     

Yield and overflow from a finite stochastic reservoir

The paper discusses the overflow (spillage) and yield rates and the total overflow and total yield over a specified time from a finite discrete stochastic reservoir, in which the yieldY _t during the working interval (t,t+1) is a function of the storageZ _t at timet, the inflow sequence {X _t } being IID.The distribution vector of the spillage rate at timet is a telescoped version of the distribution of a certain Markovian variable whose transition matrix is derived. Formulae are given for the distribution of the total spillageW _h given suitable initial conditions, forh=1,2,3; and a simple expression derived forE(W _h ).The distribution of the yield rateY _t is a trivial modification of the storage distribution. As for the total yieldR _t =Y ₁+...+Y _t , it is shown that the bivariate sequence {R _t ,Z _t } is first-order Markovian, whereZ _t is the storage at timet. The transition matrix of this process is obtained and the method of evaluating the marginal distribution of the total yieldR _t is exemplified.     

Symmetries of time-dependent magnetoconvection

Abstract

In the presence of a magnetic field, convection may set in at a stationary or an oscillatory bifurcation, giving rise to branches of steady, standing wave and travelling wave solutions. Numerical experiments provide examples of nonlinear solutions with a variety of different spatiotemporal symmetries, which can be classified by establishing an appropriate group structure. For the idealized problem of two-dimensional convection in a stratified layer the system has left-right spatial symmetry and a continuous symmetry with respect to translations in time. For solutions of period P the latter can be reduced to Z ₂ symmetry by sampling solutions at intervals of ½P. Then the fundamental steady solution has the spatiotemporal symmetry D ₂ = Z ₂ ? Z ₂ and symmetry-breaking yields solutions with Z ₂ symmetry corresponding to travelling waves, standing waves and pulsating waves. A further loss of symmetry leads to modulated waves. Interactions between the fundamental and its first harmonic are described by the group D _2h = D ₂ ? Z ₂ and its invariant subgroups, which describe solutions that are either steady or periodic in a uniformly moving frame. For a Boussinesq fluid in a layer with identical top and bottom boundary conditions there is also an up-down symmetry. With fixed lateral boundaries the spatiotemporal symmetries, again described by D _2h and its invariant subgroups, can be related to results obtained in numerical experiments and analysed by Nagata et al. (1990). With periodic boundary conditions, the full symmetry group, D _2h ?Z ₂, is of order 16. Its invariant subgroups describe pure and mixed-mode solutions, which may be steady states, standing waves, travelling waves, pulsating waves or modulated waves.     

Horizontal hydraulic conductivity of shallow streambed sediments and comparison with the grain‐size analysis results

Streambed horizontal hydraulic conductivity (K_h) has a substantial role in controlling exchanges between stream water and groundwater. We propose a new approach for determining K_h of the shallow streambed sediments. Undisturbed sediment samples were collected using tubes that were horizontally driven into streambeds. The sediment columns were analysed using a permeameter test (PT) on site. This new test approach minimizes uncertainties due to vertical flow in the vicinity of test tube and stream stage fluctuations in the computation of the K_h values. Ninety‐eight PTs using the new approach were conducted at eight sites in four tributaries of the Platte River, east‐central Nebraska, USA. The K_h values were compared with the nondirectional hydraulic conductivity values (K_g) determined from 12 empirical grain‐size analysis methods. The grain‐size analysis methods used the same sediment samples as K_h tests. Only two methods, the Terzaghi and Shepherd methods, yielded K_g values close to the K_h values. Although the Sauerbrei method produced a value relatively closer to K_h than other nine grain‐size analysis methods, the values from this method were not as reliable as the methods of Terzaghi and Shepherd due to the inconsistent fluctuation of the average estimates at each of the test sites. The Zunker, Zamarin, Hazen, Beyer, and Kozeny methods overestimated K_h, while the Slichter, US Bureau of Reclamation (USBR), Harleman, and Alyamani and Sen methods underestimated K_h. Any of these specific grain‐size methods might yield good estimates of streambed K_h at some sites, but give poor estimates at other sites, indicating that the relationship between K_g and K_h is significantly site dependent in our study. Copyright © 2011 John Wiley & Sons, Ltd.     

First Results of DSD Measurement by Videodistrometer in the Czech Republic in 1998-1999

The first results of the almost one year drop size distribution (DSD) measurement in the Czech Republic are summarised in this study. The ESA-ESTEC 2D videodistrometer was used to measure the rain drop parameters. The average DSD is shown to be of the gamma type. One minute DSDs were evaluated to test the accuracy of analytical DSD models. Parameters of gamma distribution and exponential distribution functions were evaluated for the whole data set as well as for the various rain rate intervals. Regression technique and the method of moments were applied to estimate the parameters of DSD. It is shown that the parameter value strongly depends on the method of computation as well as on the rain type. Its average value is about 0.59 for the average (smooth) one minute DSD while an average value of un-smoothed DSD is 11.0 (moment method) or 5.4 (regression technique). The Joss's shape parameter and the Tokay-Short's parameter CS estimating roughly the rain type are also discussed (if CS>1, the event should be convective). The tendency of increasing numerical value of the CS parameter with the increasing rain rate was observed (the DSDs were distributed into classes respecting the rain rate value) and thus the idea of the convectivity occurrence bounded with the higher CS parameter value was supported. The study also compares the parameters of the average DSD with the averages of parameter values of all 4 183 one minute DSDs.     

A Single-Station Spectral Model of the Monthly Median foF2 and M(3000)F2

A new single-station model (SSM) for monthly median values of the ionospheric parameters foF2 and M(3000)F2 has been developed. Fourier analysis provides a tool for decomposing the time-varying ionospheric parameters. The 12–month smoothed sunspot number R ₁₂ was used as an external solar characteristic because of its availability and predictability. However, for the first time, the solar activity is described not only by R ₁₂ , but also by the linear coefficient K _R representing the tendency of the change of solar activity. A general non-linear approximation of the influence of the solar-cycle characteristics R ₁₂ and K _R and ionospheric parameters foF2 and M(3000)F2 was accepted. The new SSM is applied to several European stations and its statistical evaluation shows better results than the other two SSMs used in the paper. The approach described in the paper does not contradict the use of different synthetic ionospheric indices (as the T-index, MF2–index); the basic aim is to show only that using one additional new characteristic of the solar-cycle variations, such as K _R , improves the monthly median model.     

Anisotropy and depth‐related heterogeneity of hydraulic conductivity in a bog peat. I: laboratory measurements

Anisotropy and heterogeneity of hydraulic conductivity (K) are suspected of greatly affecting rates and patterns of ground‐water seepage in peats. A new laboratory method, termed here the modified cube method, was used to measure horizontal and vertical hydraulic conductivity (K_h and K_v) of 400 samples of bog peat. The new method avoids many of the problems associated with existing field and laboratory methods, and is shown to give relatively precise measurements of K. In the majority of samples tested, K_h was much greater than K_v, indicating that the bog peat was strongly anisotropic. Log₁₀K_h, log₁₀K_v, and log₁₀ (K_h/K_v) were found to vary significantly with depth, although none of the relationships was simple. We comment on the scale dependency of our measurements. Copyright © 2002 John Wiley & Sons, Ltd.     

Inhibition Concentration of Phenolic Substances under Different Cultivation Conditions. Part I: Phenol Oxidation by Mixed Microbial Population in a Model System

The effects of oxygen supply rate and the presence or absence of nutrients on the kinetics of phenol degradation and oxygen consumption by a mixed microbial population were tested in a model system. The values for the maximum specific rate of phenol degradation (q_Smax), the saturation constant (K_S), and the inhibition concentration (S_CR) were determined for unlimited growth (K_La = 340 h^?1, growth medium) with 1.7 mmol g^?1 h^?1, 65 mg L^?1, and 190 mg L^?1. Under limitation conditions, alterations occur depending on the type of limitation. Nutrient limitations lead to values of 0.8 mmol g^?1 h^?1, 45 mg L^?1, and 160 mg L^?1, and oxygen limitations lead to 1.2 mmol g^?1 h^?1. 30 mg L^?1, and 120 mg L^?1, respectively. The results suggest that with excess oxygen, the rate of phenol degradation was higher and the inhibition effect of phenol was suppressed to some extent. Under the same high oxygen supply rate, the presence of nutrients in the model water significantly supported the phenol degradation rate.