1. Introduction

Salt water accounts for 97% of the global distribution of water resources, but only 3% of the fresh water, which is approximately 22% of all water on Earth, is stored as groundwater as a main obtainable resource [1]. Nevertheless, over the past half century, global warming has caused a rise in sea level that has resulted in changes in groundwater [2]. Groundwater depletion, which causes land subsidence, salt water intrusion, the necessity to irrigate agriculture, and reductions in food security, is becoming more severe year by year [3–5]. Globally, groundwater storage is much greater than that of atmospheric water and soil water. Because of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

significant differences in the storage of water resources, determining a method by which to assess groundwater storage is a very important issue [6].

was no sign of danger to water supply shortages. Hughes et al. [19] found that precipitation and changes in groundwater storage have a close connection. When annual precipitation is above the threshold, groundwater storage will increase and vice versa. Zhang et al. [20] selected 17 catchments to assess perennial groundwater storage trends. The groundwater storage showed different trends over the various shorter periods, and strong consistencies in the trends existed across most catchments. These results suggest that short-term climate variability can significantly influence groundwater storage and that it will be affected by largescale hydro-climate factors. A better insight into how groundwater storage has been changing will be critical in evaluating sustainable water resource management plans. The objectives of this study are (1) to discuss the relationship between discharge and storage and (2) to assess groundwater storage dynamics using low flow analysis to assess what water resource prob-

The Discharge-Storage Relationship and the Long-Term Storage Changes of Southern Taiwan

http://dx.doi.org/10.5772/intechopen.73163

57

In a catchment, a hydrological system can be represented with the mass conservation equation:

Assuming that discharge is only related to storage, f(S) can be expressed as the storage-

Assuming that Q is an increasing single-valued function of S since storage-discharge function

Q is assumed to be an increasing single-valued function of S. Additionally, S cannot be directly

S ¼ f �1

The discharge change rate through time is yielded by combining Eqs. (1) and (2)

dS can be defined as a function of Q

dS dt <sup>¼</sup> dQ

dQ dt <sup>¼</sup> dQ dS

dQ dS <sup>¼</sup> <sup>f</sup> 0 ð Þ¼ S f 0 f �1

dt <sup>¼</sup> <sup>P</sup> � <sup>E</sup> � <sup>Q</sup> (1)

] is the unit area discharge.

Q ¼ f Sð Þ (2)

ð Þ Q (3)

dS ð Þ <sup>P</sup> � <sup>E</sup> � <sup>Q</sup> (4)

ð Þ <sup>Q</sup> <sup>¼</sup> <sup>g</sup>ð Þ <sup>Q</sup> (5)

] is the rate of precipita-

dS

] is the evapotranspiration rate; and Q [LT�<sup>1</sup>

] is the unit area water stored in the catchment; P [LT�<sup>1</sup>

lems southern Taiwan will be facing.

2. Methodology

where S [LT�<sup>1</sup>

discharge relationship

measured, and thus dQ

tion; E [LT�<sup>1</sup>

is invertible

2.1. Water balance concept

Recently, Taiwan has been affected by climate change, which is resulting in distinct wet and dry seasons with stronger rainfall intensity in the wet season and a lack of rain in the dry season [7]. Variations in Taiwan's topography and uneven distribution of rainfall, with more in the mountains than in the plains and more in the northern than in the southern areas, are causing uneven distribution of water resources in time and space. Taiwan's annual average rainfall is up to 2500 mm, which is approximately 2.5 times more than the annual global average rainfall. Nevertheless, Taiwan is still facing a surface water shortage problem in the dry season. In southern Taiwan, which is the most serious area, the ratio of wet season rainfall and dry season rainfall is as high as 9:1 [8]. Therefore, in this study, the southern area is set as the study area to discuss what groundwater resource problems this area will be facing in the future. The water balance concept is used to explore the storage-discharge relationship, and a low flow analysis is used to assess the lowest groundwater storage in southern Taiwan. The assessment results can be provided to water resource agencies to assist with water resource management plans.

Low flow analysis, which is a hydrological method widely used to estimate hydraulic parameters, is used to characterize basin characteristics and assess groundwater storage trends. Brutsaert and Nieber [9] analyzed six basins in the Finger Lakes region of upstate New York using the lower envelope fitting method to characterize recession curves in order to estimate specific yield. Szilagyi et al. [10] pointed out that complexity of basin shape, hydraulic conductivity heterogeneity, and gently sloping impermeable layers do not affect the estimation of hydraulic conductivity and mean aquifer depth. Field-based estimates of these hydraulic parameters using a low flow analysis are a beneficial method for evaluating discharge data that are not easily obtained, mainly in low population density area [11].

Watershed properties such as hydrology, geology, and topography affect streamflow regimes [12]. Thus, through knowing the main factors dominating streamflow recession characteristics, it is possible to understand drainage systems. Zecharias and Brutsaert [13] selected 19 basins in the Allegheny Mountains of the Appalachian Plateaus to investigate streamflow recession curve characteristics, and their results showed that recession time is affected by mean basin slope, drainage density, and the ratio of the hydraulic conductivity and the specific yield. Vogel and Kroll [14] evaluated Massachusetts basin characteristics, and their result showed that recession time is highly correlated with basin area and basin slope. Brutsaert [15] suggested that streamflow recession time is highly correlated with basin area, basin elevation, and stream length. Kirchner [16] used a single-equation rainfall-runoff model to select streamflow data in order to estimate recession time from the headwaters of the Severn and Wye rivers at Plynlimon in mid-Wales. Sánchez-Murillo et al. [17] suggested that geology affects the length of recession time. Metamorphic and sedimentary rocks result in longer recession time. Low flow in flat basalt landscapes recesses rapidly. Knowing the dominating factors related to recession characteristics makes it easier to understand their effects on subsurface properties.

Recently, several studies analyzed annual groundwater storage trends in order to assess water supply availability in the future. Sugita and Brutsaert [18] researched perennial groundwater storage and low flow trends in the Kanto region. If the water demand remained constant, there was no sign of danger to water supply shortages. Hughes et al. [19] found that precipitation and changes in groundwater storage have a close connection. When annual precipitation is above the threshold, groundwater storage will increase and vice versa. Zhang et al. [20] selected 17 catchments to assess perennial groundwater storage trends. The groundwater storage showed different trends over the various shorter periods, and strong consistencies in the trends existed across most catchments. These results suggest that short-term climate variability can significantly influence groundwater storage and that it will be affected by largescale hydro-climate factors. A better insight into how groundwater storage has been changing will be critical in evaluating sustainable water resource management plans. The objectives of this study are (1) to discuss the relationship between discharge and storage and (2) to assess groundwater storage dynamics using low flow analysis to assess what water resource problems southern Taiwan will be facing.
