Agricultural Drainage Basin Research Articles

A unified model for high resolution mapping of global lake (>1 ha) clarity using Landsat imagery data

Lake clarity, usually measured by Secchi disc depth (SDD), is a reliable proxy of lakes trophic status due to its close link with total suspended matter, chlorophyll-a, and nutrients. Trained with in-situ measured SDD and match-up Landsat images, we established various regression models to estimate SDD for global lakes. We selected a unified model which demonstrated good spatiotemporal transferability, and has potential to map SDD in different years with good quality of Landsat top-of-atmosphere (TOA) images embedded in Google Earth Engine (GEE). The unified model was successfully calibrated (n = 3586 data points, R2 = 0.84, MAPE = 29.8%) against SDD measured in 2235 lakes across the world, and the validation (n = 1779, R2 = 0.76, MAPE = 38.8%) also exhibited stable performance. The unified model was tuned to historical SDD measurements coincident with different Landsat sensors (L5-TM, L7-ETM+, L8-OLI) launched over the past four decades (1984–2020), thus confirming its temporal stability. Global SDD was mapped using GEE with OLI TOA products mainly acquired in 2019 to examine the spatial variation of lake water clarity (lake surface area ≥ 1 ha) all over the world. Worldwide, lake water clarity averaged 3.13 ± 1.71 m in 2019, but exhibited remarkable spatial variability due to catchment hydrological and landscape settings, lake morphology, elevation and anthropogenic impact. Inland waters in Europe (4.18 ± 1.82 m) and North America (3.84 ± 1.77 m) had the highest clarity due to greater water depth combined with less human disturbance in the high latitude regions. Lakes in South America (2.50 ± 2.33 m), Asia (2.44 ± 1.63 m) and Africa (2.36 ± 0.72 m) displayed intermediate clarity. Lakes in Oceania (1.97 ± 1.48 m) exhibited the lowest clarity for all continents except Antarctica. Further, we used the mapped SDD to evaluate water trophic status using the Carlson trophic state index. Our results indicate that, in 2019, about 63.6% of the lake areas and 47.8% of total lake numbers (2,219,627/4,646,056) were oligotrophic for global lakes, while about 23.6% areal percent and 37.1% of lake numbers are eutrophic mostly as a result of their being located in agricultural and urban-dominated drainage basins. This study, for the first time, provides water clarity information for lakes with area ≥ 1 ha all over the world with 30-m resolution and facilitates the understanding of the water clarity relevant to TSM (r = 0.95), Chl-a (r = 0.73), total phosphorus (r = 0.75), total nitrogen (r = 0.60), which could further provide water clarity data and technical support for trophic level evaluations as well. This unified model could serve as a powerful research tool for long-term monitoring of aquatic ecosystems and assessing their resilience to anthropogenic disturbance and climate change-related stressors.

Advances in the simulation of nutrient dynamics in cold climate agricultural basins: Developing new nitrogen and phosphorus modules for the Cold Regions Hydrological Modelling Platform

Excess nutrients in aquatic ecosystems is a major water quality problem globally. Worsening eutrophication issues are notable in cold temperate areas, with pervasive problems in many agriculturally dominated catchments. Predicting nutrient export to rivers and lakes is particularly difficult in cold agricultural environments because of challenges in modelling snow, soil, frozen ground, climate, and anthropogenic controls. Previous research has shown that the use of many popular small basin nutrient models can be problematic in cold regions due to poor representation of cold region hydrology. In this study, the Cold Regions Hydrological Modelling Platform (CRHM), a modular modelling system, which has been widely deployed across Canada and cold regions worldwide, was used to address this problem. CRHM was extended to simulate biogeochemical and transport processes for nitrogen and phosphorus through a complex of new process-based modules that represent physicochemical processes in snow, soil and freshwater. Agricultural practices such as tillage and fertilizer application, which strongly impact the availability and release of soil nutrients, can be explicitly represented in the model. A test case in an agricultural basin draining towards Lake Winnipeg shows that the model can capture the extreme hydrology and nutrient load variability of small agricultural basins at hourly time steps. It was demonstrated that fine temporal resolutions are an essential modelling requisite to capture strong concentration changes in agricultural tributaries in cold agricultural environments. Within these ephemeral and intermittent streams, on average, 30%, 31%, 20%, and 16% of the total annual load of nitrate (NO3), ammonium (NH4), soluble reactive phosphorus (SRP), and particulate phosphorous (partP)NO3, NH4, SRP and partP occurred during the episodic snowmelt freshet (∼9 days, accounting for 21% of the annual flow), but shows extreme temporal variation. The new nutrient modules are critical tools for predicting nutrient export from small agricultural drainage basins in cold climates via better representation of key hydrological processes, and a temporal resolution more suited to capture dynamics of ephemeral and intermittent streams.

Assessing water quality changes in agricultural drainages: Examples from oxbow lake tributaries in northwestern Mississippi, United States, and simulation-based power analyses

Hydrology (streamflow and stage) and water quality (suspended sediment, total nitrogen [N], ammonia [NH₃], total Kjeldahl N, nitrate plus nitrite [NO₃ + NO₂], and total phosphorus [TP]) were monitored in two small agricultural drainages in northwestern Mississippi to document changes in water quality that coincided with the implementation of best management practices (BMPs) in upstream drainages. Using an event-based data set and bootstrapping techniques, we tested for difference and equivalence in median event concentration and differences in concentration-streamflow (C-Q) relationships between an early and late period at each site, where most of the major BMP implementation occurred during the early period. Results for Bee Lake Tributary were inconclusive. Using 95% confidence intervals, none of the constituents were statistically different or equivalent for median event concentrations between the periods, indicating a lack of evidence to determine whether water quality had changed or stayed the same, and only TP had a significantly higher C-Q intercept during the late period. At Lake Washington Tributary, more than half the constituents had a significantly different median, slope, or intercept between periods, indicating a 35% or more decrease in event concentration following a period of intense BMP implementation. These mixed results could be due to a variety of differences between the sites including the type and timing of BMP implementation, production practices, and crop types. We also used the monitoring data to generate synthetic data and performed a simulation-based power analysis to explore the ability to detect change under 25 scenarios of sampled event counts and hypothetical percentage changes. The simulation-based power analysis indicated that natural variability in event concentration and flow hindered our ability to detect change. The results from this study can be used to guide future decisions pertaining to monitoring efforts in small agricultural drainage basins in the Mississippi Alluvial Plain.

Comparison of Acoustic to Optical Backscatter Continuous Measurements of Suspended Sediment Concentrations and Their Characterization in an Agriculturally Impacted River

The increased soil loss in an agricultural watershed raises challengers for river water quality and a reliable automated monitoring for suspended sediment concentrations (SSC) is crucial to evaluate sediment budgets variation in systems. The aims of this study were (1) to test if an acoustic doppler current profiler (ADCP) would give similar results to turbidity probe measurements as a high frequency monitoring tool for suspended sediment; and (2) to analyze the relationship between sediment drivers and SSC in a typical agricultural drainage basin. The acoustic and optical backscatter sensors were used to collect SSC data during the ice-free seasons of four consecutive years in the Dunk River (PEI, Canada). The slopes of the relationships between the two SSC indirect measurements were not significantly different than 1. Correlations between SSC and hydro-meteorological variables showed that the high SSC values were more associated with the streamflow and water velocity than precipitation. This study highlighted the great potential of ADCP for the continuous monitoring of suspended sediment in an agricultural watershed. For summer periods the prevalence of clockwise hysteresis (74.1% of measured rainstorm events with SSC > 25 mg L−1) appeared related to rainstorm behaviors.

Effects of pasture conversion to sugarcane for biofuel production on stream fish assemblages in tropical agroecosystems

Abstract One of the main causes of biodiversity loss in aquatic ecosystems is instream habitat change associated with land‐use change in the drainage basin. Biofuel crop production is an increasingly important driver of land‐use change in existing agricultural drainage basins. Conceptually, biofuels such as sugarcane (Saccharum spp.: Poaceea) can provide a sustainable alternative to fossil fuels, but little is known about the consequences of biofuel crop production for biodiversity. Thus, our aim was to investigate if and how the degree of further environmental changes at regional and local scales can influence taxonomic and functional structure of fish assemblages in agroecosystem streams. We quantified drainage basin land use, instream and riparian habitat, and fish assemblages in 38 agroecosystem streams in Brazil with drainage basins experiencing rapid expansion of sugarcane production, and calculated degree of environmental change and corresponding response in taxonomic and functional structure of stream fish assemblages. Procrustes and redundancy analyses tested for correlation between land use and instream habitat and environmental factors correlated with fish assemblage structure, respectively. Previous studies demonstrated relationships between land‐use change, instream habitat change and taxonomic and functional structure of stream fish assemblages. Therefore, we used regression to test the hypothesis that degree of environmental change is positively correlated with magnitude of change in taxonomic and functional structure of fish assemblages. Primary gradients of environmental change included conversion of pasture to sugarcane and reduction of stream width, depth and volume. Although drainage basin and instream attributes were not correlated within a year, we detected a legacy effect of land use on instream habitat. The degree of environmental change was not significantly correlated with taxonomic or functional changes in stream fish assemblages in most cases. However, abundances of resistant taxa increased, whereas nektonic functional groups decreased, and the biological response also presented evidence of a legacy effect. Our findings demonstrate that instream habitat and aquatic biodiversity in streams with agricultural drainage basins undergoing rapid conversion to biofuel production likely will not experience the full magnitude of responses in the short term. Therefore, repeated sampling of sufficient duration to account for legacy effects or time lags is needed to test for effects of biofuel expansion on biodiversity, and similarly to assess whether management practices such as riparian zone preservation can benefit biodiversity in agroecosystems.

Influence of an intensive agricultural drainage basin on the seasonal distribution of organic pollutants in seawater from a Mediterranean coastal lagoon (Mar Menor, SE Spain)

The seasonal distribution of polycyclic aromatic hydrocarbons, polychlorinated biphenyls, organochlorine pesticides, organophosphorus pesticides, triazines and other organic pollutants in surface seawater from the Mar Menor lagoon (SE Spain) was characterized from spring 2009 to winter 2010 by stir bar sorptive extraction and thermal desorption followed by capillary gas chromatography coupled to mass spectrometry (SBSE–GC–MS). The most ubiquitous pollutants were chlorpyrifos, chlortal-dimethyl, terbuthylazine, naphthalene and propyzamide throughout the year. Insecticides and herbicides were predominant in summer and spring, and fungicides were more abundant in autumn and winter. Chlorpyrifos was predominant in autumn and spring; herbicides (propyzamide, terbuthylazine and terbuthylazine-desethyl) in autumn and fungicides (flutolanil, boscalid, cyprodinil) in autumn and winter. The highest concentrations for the majority of pollutants were detected in autumn as consequence of air and surface/ground water inputs, and above all of desorption from terrigenous materials deposited in sediments by two intense flash flood events (September 2009).

Analysis of Saccharomyces cerevisiae null allele strains identifies a larger role for DNA damage versus oxidative stress pathways in growth inhibition by selenium

Selenium toxicity is a growing environmental concern due to widespread availability of high-dose selenium supplements and the development of high-selenium agricultural drainage basins. To begin to analyze the effects of selenium toxicity at the genetic level, we have systematically determined which genes are involved in responding to high environmental selenium using a collection of viable haploid null allele strains of Saccharomyces cerevisiae representing three major stress pathways: the RAD9-dependent DNA repair pathway, the RAD6/RAD18 DNA damage tolerance pathway, and the oxidative stress pathway. A total of 53 null allele strains were tested for growth defects in the presence of a range of sodium selenite and selenomethionine (SeMet) concentrations. Our results show that approximately 64-72% of the strains lacking RAD9-dependent DNA repair or RAD6/RAD18 DNA damage tolerance pathway genes show reduced growth in sodium selenite versus approximately 28-36% in SeMet. Interestingly both compounds reduced growth in approximately 21-25% of the strains lacking oxidative stress genes. These data suggest that both selenite and SeMet are likely inducing DNA damage by generating reactive species. The anticipated effects of loss of components of the oxidative stress pathway were not observed, likely due to apparent redundancies in these gene products that may keep the damaging effects in check.

Hysteresis effect in suspended sediment concentration in the Rybárik basin, Slovakia / Effet d'hystérèse dans la concentration des sédiments en suspension dans le bassin versant de Rybárik (Slovaquie)

Suspended sediment dynamics influenced by rainstorms and factors controlling changes in suspended sediment concentrations, were investigated during hydrological events in a small scale, since small agricultural drainage basins can be considered as one of the most important sediment sources. Suspended sediment concentrations were measured during discharge waves in the years 1987–1990, 1992, 2000 and 2001. Positive and anti-clockwise hysteresis was observed at Rybárik basin. A procedure of isolating factors controlling suspended sediment concentrations and dynamics has given a more realistic view on production and transport of suspended sediment. It is confirmed that spatial and temporal variability of sediment availability and suspended sediment dilution by the baseflow, mainly in the case of two or more waves immediately following one after another, significantly control suspended sediment concentrations and dynamics.

Trace metals in tissues of resident and migratory birds from a lagoon associated with an agricultural drainage basin (SE Gulf of California).

With the aim of knowing Cd, Cu, Fe, Mn, Pb, and Zn concentrations in selected tissues of birds from two places at Altata-Ensenada del Pabellón lagoon in the SE Gulf of California, 39 specimens of 14 species were analyzed. Migratory birds in this study showed the highest values of Cd, Cu, and Pb in liver; Fe and Mn, in viscera; and Zn, in feathers. Concerning the resident avifauna, the highest levels of Cd, Cu, and Fe were detected in the liver, Mn in viscera; and Pb and Zn, in feathers. Regarding Cu and Fe, higher concentrations were detected in migratory avifauna, while Mn and Zn were more accumulated in resident waterfowl. In the case of Cd and Pb, both elements showed a tendency to be more accumulated in resident seabirds. Statistical comparisons showed that in carnivorous and omnivorous birds the resident component was the group with higher levels in more comparisons where mainly Cu and Fe were involved. In the rest of the metals the differences in the concentrations in the distinct tissues of migratory versus resident species were not clearly evident in one group in particular. In a few cases, carnivorous birds had higher levels of Cd than herbivorous and omnivorous birds; for the rest of the metals there was not a clear trend of metal accumulation.

Integrated hydrological modelling of a managed coastal Mediterranean wetland (Rhone delta, France): initial calibration

Abstract. This paper presents a model of a heavily managed coastal Mediterranean wetland. The hydrosystem studied , called ``Ile de Camargue", is the central part of the Rhone river delta. It comprises flat agricultural drainage basins, marshes, and shallow brackish lagoons whose connection to the sea is managed. This hydrosystem is subject to strong natural hydrological variability due to the combination of a Mediterranean climate and the artificial hydrological regime imposed by flooded rice cultivation. To quantify the hydrological balance at different spatial and temporal scales, a simplified model is developed — including the basin and the lagoons — using a time step that enables the temporal dynamic to be reproduced that is adapted to data availability. This modelling task takes into account the functioning of the natural and anthropogenic components of the hydrosystem. A conceptual approach is used for modelling drainage from the catchment, using a GIS to estimate water input for rice irrigation. The lagoon system is modelled using a two-dimensional finite element hydrodynamic model. Simulated results from the hydrodynamic model run under various hydro-climatic forcing conditions (water level, wind speed and direction, sea connection) are used to calculate hydraulic exchanges between lagoon sub units considered as boxes. Finally, the HIC ("Hydrologie de l’Ile de Camargue") conceptual model is applied to simulate the water inputs and exchanges between the different units, together with the salt balance in the hydrosystem during a calibration period. Keywords: water management,conceptual hydrological model, hydrodynamic model, box model, GIS, Rhone delta, Camargue.

Stream Invertebrate Communities, Water Quality, and Land-Use Patterns in an Agricultural Drainage Basin of Northeastern Nebraska, USA.

We used invertebrate bioassessment, habitat analysis, geographic information system analysis of land use, and water chemistry monitoring to evaluate tributaries of a degraded northeast Nebraska, USA, reservoir. Bimonthly invertebrate collections and monthly water chemistry samples were collected for two years on six stream reaches to identify sources contributing to reservoir degradation and test suitability of standard rapid bioassessment methods in this region. A composite biotic index composed of seven commonly used metrics was effective for distinguishing between differentially impacted sites and responded to a variety of disturbances. Individual metrics varied greatly in precision and ability to discriminate between relatively impacted and unimpacted stream reaches. A modified Hilsenhoff index showed the highest precision (reference site CV = 0.08) but was least effective at discriminating among sites. Percent dominance and the EPT (number of Ephemeroptera, Plecoptera, and Trichoptera taxa) metrics were most effective at discriminating between sites and exhibited intermediate precision. A trend of higher biotic integrity during summer was evident, indicating seasonal corrections should differ from other regions. Poor correlations were evident between water chemistry variables and bioassessment results. However, land-use factors, particularly within 18-m riparian zones, were correlated with bioassessment scores. For example, there was a strong negative correlation between percentage of rangeland in 18-m riparian zones and percentage of dominance in streams (r(2) = 0.90, P < 0.01). Results demonstrate that standard rapid bioassessment methods, with some modifications, are effective for use in this agricultural region of the Great Plains and that riparian land use may be the best predictor of stream biotic integrity.

Sediment export by water from an agricultural catchment in the Loam Belt of central Belgium

An agricultural drainage basin of 250 ha was selected in the Belgian Loam Belt to evaluate sediment export by water. Water discharge was continuously measured at the outlet of the catchment and suspended sediment samples were taken proportional with discharge during rainfall events. From these samples supended sediment concentration and grain-size distribution were determined. Within the catchment, regular surveys were conducted in order to obtain detailed data on vegetation cover, soil surface parameters and erosion and sedimentation features. Important variations in sediment yield were detected at various time-scales, ranging from a year, over a season to one event. These variations could be linked with processes occurring within the catchment. Due to the development of a vegetation cover, the suspended sediment concentrations were lower during the summer period than during the winter period for the same discharge. The evolution in both suspended sediment concentration and grain-size distribution during the winter period is explained by variations in the soil surface state of the fields as well as the development of a rill and gully network. The positive hysteresis observed within a single event is explained by taking into account the distance between the sediment source and the catchment outlet and not by sediment flushing. Our analysis shows that the role of the main factors controlling the geomorphic response of such a catchment can be identified only if measurements at the outlet are complemented by detailed data on the conditions within the catchment. Catchment model calibration and validation are therefore impossible if this information is lacking.

The spatial variability of Chernobyl-derived [formula omitted] inventories in a small agricultural drainage basin in central Russia

Little information currently exists regarding the small-scale spatial variability of Chernobyl radiocaesium fallout and associated inventories. This contribution reports the results of a study of the variability of 137 Cs inventories within the 2.18-km 2 Lapki balka catchment located near Tula in central Russia. The local area was characterized by 137 Cs inventories in excess of 200 kBq m −2 immediately after the Chernobyl accident and pre-existing bomb-derived inventories can be ignored in view of their very low magnitude. Field sampling and measurements included both collection of soil cores for subsequent laboratory analysis and in situ field measurements using a CORAD portable detector. The results obtained show evidence of a systematic south–north increase in the reference inventory across the basin, which must be taken into account when interpreting subsequent radiocaesium redistribution within the basin. Random spatial variability of 137 Cs inventories of a similar magnitude to that reported for bomb-derived fallout was also documented. The extent of random spatial variability varied between different geomorphological units. Maximum variability, with coefficients of variation up to 20%,was associated with areas of sediment accumulation within the balka bottoms. Substantial variability (cv. typically ca. 15%) was found within flat cultivated areas and undisturbed areas both on the interfluves and on the balka sides, all of which could serve as reference sites. Minimum variability (cv. typically ca. 12%) was associated with the cultivated slopes with no evidence of sediment accumulation.

Fish Community Structure as a Measure of Degradation and Rehabilitation of Riparian Systems in an Agricultural Drainage Basin

/ Assessing the health of ecological components of agroecosystems may be accomplished by examining changes in the drainage basin, which serves as an integrator of the agroecosystem landscape. In this study we examined fish communities in terms of an array of indicators of structure and related these to changes in riparian vegetation and agricultural practice. Evidence suggests management practices designed to foster healthier environments by, for example, reestablishing riparian vegetation were associated with positive impacts on the integrity of the fish community. At the same time, continued intensification of agricultural practices in parts of the drainage basin in recent years likely has had an off-setting influence in overall improvements in agroecosystem health. Assessments of changes in the structure of the fish associations provide the balance sheet by which the counteracting influences can be aggregated and assessed.KEY WORDS: Fish community structure; Riparian system; Agricultural drainage basin

DETECTING CILANGES IN THE SPATIAL DISTRIBUTION OF NITRATE CONTAMINATION IN GROUND WATER1

ABSTRACT: Many studies of ground water pollution in general and nitrate contamination in particular have often relied on a one‐time investigation, tracking of individual wells, or aggregate summaries. Studies of changes in spatial distribution of contaminants over time are lacking. This paper presents a method to compare spatial distributions for possible changes over time. The large‐scale spatial distribution at a given time can be considered as a surface over the area (a trend surface). The changes in spatial distribution from period to period can be revealed by the differences in the shape and/or height of surfaces. If such a surface is described by a polynomial function, changes in surfaces can be detected by testing statistically for differences in their corresponding polynomial functions. This method was applied to nitrate concentration in a population of wells in an agricultural drainage basin in Iowa, sampled in three different years. For the period of 1981–1992, the large‐scale spatial distribution of nitrate concentration did not show significant change in the shape of spatial surfaces; while the magnitude of nitrate concentration in the basin, or height of the computed surfaces showed significant fluctuations. The change in magnitude of nitrate concentration is closely related to climatic variations, especially in precipitation. The lack of change in the shape of spatial surfaces means that either the influence of land use/nitrogen management was overshadowed by climatic influence, or the changes in land use/management occurred in a random fashion.

A study on the effect of spatial scale on the parameters of a sediment transport equation for sheetwash

Event-based prediction of soil erosion over large agricultural fields and drainage basins is limited by the lack of data for the relevant scales of applications. Thus, researchers have to assume that the parameters of the sediment transport equation obtained from very small scale can be transferred to large scale systems. This assumption remains untested. The objectives of this paper are: (1) to illustrate, by a compilation of a wide range of experiments on sheetwash erosion, the lack of investigations conducted on an event by event basis over agricultural fields; (2) to present a synthesis of results obtained during three experiments conducted for short duration events at various spatial scales ranging from 0.25 to 55 meters. The results will show the possibility of using a relation calibrated for a small scale experimental plot on a larger scale agricultural field under specific conditions.

Heavy metals in clams from a subtropical coastal lagoon associated with an agricultural drainage basin.

Heavy metals in clams from a subtropical coastal lagoon associated with an agricultural drainage basin.

Heavy metals in oysters from a subtropical coastal lagoon associated with an agricultural drainage basin

Heavy metals in oysters from a subtropical coastal lagoon associated with an agricultural drainage basin

Evaluation of the accuracy and precision of annual phosphorus load estimates from two agricultural basins in Finland

The accuracy and precision of phosphorus load estimates from two agricultural drainage basins in western Finland were evaluated, based on continuous flow measurements and frequent flow-proportional sampling of total phosphorus concentration during a 2 year period. The objective was to compare different load calculation methods and to evaluate alternative sampling strategies. An hourly data set of concentrations was constructed by linear interpolation, and these data were used in Monte Carlo runs for producing replicate data sets for calculating the accuracy and precision of load estimates. All estimates were compared with reference values computed from the complete hourly data sets. The load calculation methods based on summing the products of regularly sampled flows and concentrations produced the best precision, whereas the best accuracy was achieved using methods based on multiplying annual flow by flow-weighted annual mean concentration. When comparing different sampling strategies, concentrating sampling in high runoff periods (spring and autumn) was found to give better accuracy and precision than strategies based on regular interval sampling throughout the year. However, the best result was obtained by taking samples flow-proportionally within the highest peak flows plus additional regular interval (e.g. biweekly) samples outside these flow peaks. Using this strategy, which calls for automatic sampling equipment, accuracies better than 5% and precisions better than 10% can be achieved with only 30–50 samples year −1.

Effect of snow and soil frost melting on the concentrations of suspended solids and phosphorus in two rural watersheds in Western Finland

Hydrological processes are known to have a considerable effect on nutrient transport from agricultural land to watercourses. In cold temperate regions peak discharges are caused not only by storm conditions but also by melting of snow and frost. The objective of this work was to investigate the effects of snow and frost melt on concentrations of phosphorus and suspended solids. The samples were taken using flowweighted automatic sampling techniques from two agricultural drainage basins. During the beginning of the snowmelt period the concentration of suspended solids was rather low by comparison with the total phosphorus concentration and the discharge. The different behaviour compared with the relationships found during storm conditions was probably caused by continuous extraction of the soil surface by low ionic concentration melt water, and to some extent by leaching from plant residues. The gradual decrease of total phosphorus concentration during the frost thawing period could be attributed to the gradually increasing proportion of the subsurface and ground water discharge in the runoff.