Hydrograph recession analysis methods and its comparison using unsaturated moisture movement model

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Abstract: Groundwater plays an important role in feeding springs and streams, supporting wetlands and land surface stability. In Finland, most water is held in the soil than the surface systems. Hence, Finland’s water resources depend on groundwater and biogeochemical processes. The study of groundwater in peatland is important for maintaining ecological balance and conservation of water resources. The groundwater level is one of the key indicators of aquifer conditions and groundwater basins. It helps to interpret hydrogeology, groundwater flow, groundwater sustainability and land usability. The study tries to analyze ground water recharge on peatland catchments using hydrograph recession analysis. The equation for the hydrograph recession curve can be utilized to predict groundwater recharge during each recession period. The steps involved during recession curve analysis includes selection of analytical expression, derivation of recession characteristic and optimization of the parameters. While computing groundwater recharge with recession curve, the high variability of each recession segments creates major problem. Each segment shows the outflow process which creates short-term or seasonal influence. The variation in rate of recession which causes problems for derivation of recession characteristics. The computer software such as hydro-office, VBA macro excel and Matlab are used for recession analysis. The results obtained do not consider climatic influences. The results were then confirmed by using water balance model and statistical tests. The e-water toolkit is used for water balance model and statistical tests are performed using R-software. The rainfall-runoff data are used as input to the software used in each method. From the analysis, required output recession parameters are obtained for further calculation. These estimated recession parameters can be used to predict low flows (groundwater contribution to runoff) to understand catchment groundwater resources and as inputs for the rainfall-runoff model analysis. Hence, the objective of this study is to analyze groundwater recharge by studying the recession limb of the runoff hydrograph. The study work compares various recession analysis methods. It also tries to identify the better method by comparing groundwater recharge from different methods with groundwater recharge from unsaturated water balance model. Furthermore, the recession constants obtained from different methods are compared with the theoretical values. Statistical tests are used to identify the best method among recession analysis methods used in this study.

Keywords: Runoff Hydrograph, recession Parameters, Runoff, Groundwater recharge, unsaturated water balance, Statistical tests

1 Introduction

Peatlands are major important part of global ecosystem. It shows significant interaction with natural hydrological system, biogeochemical cycling and terrestrial as well as aquatic biodiversity. In Finland, peatlands have high influence in ecological as well as socio-economic aspects. It covers one-third of Finnish land area which is 2.0 million ha of 9.3 million ha [1]. The hydrological study is used to develop the functions and process related peatlands system. Hydrological study is an important part of environmental and ecological study in Finland. In peatlands as in other soil formation there is interactive connection between the surface and subsurface hydrological water system. This study intends to calculate yearly groundwater recharge of two catchments using recession hydrograph. It includes study of various hydrograph recession analysis methods. It also includes various climatic factors that influence runoff hydrograph. The amount of water received by catchment is disintegrated in different time period. The hydrological features of catchment influences runoff and water storage in the catchment. The runoff generated is highly influenced by upslope contributions from surface flow as well as interflow. The study of hydrological behavior in surface and subsurface of two peatlands catchment is the major objective of this study.

2 Site Description

The two catchments studied in this study are Marjasuo and Röyvänsuo with four years data (2010-2013). Marjasuo peatland has been drained since 1968 for forestry and was restored in 2011.Röyvänsuo is a pristine peatland located in Isosyöte National park. Both of the study catchments are the part of larger Iijoki catchment [2]. The catchments lie in northern Finland at Taivalkoski municipality and both are state owned. The geographical locations of the catchments Marjasuo and Röyvänsuo are at 65o48’19.79’’ latitude and 27o48’42.246’’longitude and 65o49’12.213’’ latitude and 27o48’13.978’’ longitude respectively. Marjasuo covers land area of 65ha (0.65km²) and Röyvänsuo 75ha (0.75km²). The two catchments contain almost similar terrestrial and soil formations. Marjasuo has 2.27 ha (3.5%) open water or pond, 30.55 ha (47%) mineral soil, 16.5 ha (25.5%) fen or open mire and 15.6 ha (24%) forested Peatland and paludified forest. Similarly, Röyvänsuo contains 0.5 ha (<1%) open water, 44.25 ha (59%) mineral soil, 18.75 ha (25%) fen (open mire) and 11.25 ha (15%) forested peatland and paludified forest. The map with location of two catchments are shown in Fig. (1)

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Fig. 1. Catchment location map [3].

3 Methods

The recession constants and groundwater recharge of both sites Marjasuo and Röyvänsuo are computed using the recession analysis method and specific yield method.In this study the groundwater recharge obtained from unsaturated water balance model is considered to be precise. So, the groundwater recharge obtained from recession analysis methods and specific yield are compared to groundwater recharge from unsaturated water balance model. Finally, statistical tests are carried out to observe the significance of the results obtained.The basic approaches used for this study are discussed in the following sections.

3.1 Hygrograph recession analysis

3.1.1 Individual Recession Analysis

The analysis of individual recession curves is carried out using RC 4.0 tool from hydro office software [4].By using RC tool, an individual recession segments are separated from runoff hydrograph. The runoff data is the major input and rainfall is optional. An individual recession curve is selected for short time period with small numbers of declining runoff values. In an individual analysis there is different flow constant for the slow and fast runoff. It consists of two linear models. One represents fast flow and the other represents slow flow. For each model, recession curve is divided into two portions (upperand lower).The initial flow and constant (k) are given by user. For the calibration of individual recession curve, a tool called single calibration in hydro office software is used. The outputs obtained from the software are initial flow, recession coefficient and recession time days. The output obtained is used to calculate final discharge and groundwater recharge during recession period.

i) Final runoff at time t

The output contains two initial flows and two constants for same time period. The initial flows and recession constants are added to get total flow and total constant for each individual recession curve. From total initial flow and total recession constant final discharge is calculated. The flow at the end of recession period is given by equation (1) [5]:

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Where Qt is runoff at the end of recession period (m3/s) per unit area

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The runoff and runoff time to complete one log cycle is given by equation (2) and (3) [6]:

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The calculated value t1 is used to find the groundwater recharge between each recession curve. In this method groundwater change is calculated based on each log cycle. Each individual volume is added to get total groundwater recharge volume. Individual ground water recharge volume in each log cycle is the difference between total potential groundwater runoff at beginning of recession and total groundwater potential at the end of recession.

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