Rainfall Regime and Its Impact on Water Resources on Ly Son Island, Central Vietnam45250

Based on time series data of rainfall collected during the period of 1985 - 2015 of the Ly Son Marine Meteorological island Station 15023’N;109009’E, the authors have calculated all the

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Rainfall Regime and Its Impact on Water Resources

on Ly Son Island, Central Vietnam

Bui Xuan Thong (1)(*) , Nguyen Van Dan 2 , Nguyen Ngoc Ha 3 , Van Phu Hung 1

(1) Institute of Oceanography and Environment, Hanoi, Vietnam

(2) Institute of Resources and Environmental Water, Hanoi, Vietnam

(3) National Center for Water Resources Planning and Investigation, Hanoi, Vietnam

* Correspondence: buixuanthonghunre@gmail.com

Abstract: Ly Son island have a special economic and national security role in Vietnam However, in recent years, Ly Son faces a shortage of fresh water and salinity intrusion due to the overexploitation

of underground water This article is the latest research on the rainfall regime and its impact on water resources on Ly Son island Based on time series data of rainfall collected during the period of 1985 -

2015 of the Ly Son Marine Meteorological island Station (15023’N;109009’E), the authors have calculated all the statistical values of rainfall such as maximum rainfall, maximum of daily and monthly precipitation and its tendency for the various periods of series data between 1985 - 2015 Using water balance equations and results computing of all maximum values of rainfall, the authors have computed all surface runoff, respectively The mean annual precipitation for the period of 1985

- 2015 was 2278.8 mm In the rainy months, the rainfall was 1779.6 mm occupied 78.1% of the whole year Over the area of Ly Son island of 10.4 km2 the estimated potential yield of rainfall was 14.17 x

106 m3/year while for the whole year, Ly Son island needs only 6.422 x 106 m3 of fresh water From that we can suggest that if there are appropriate measures for collecting rainfall in rainy season and all the days having abnormal rainfall the scarcity of water resource will be ameliorated on Ly Son island

Keywords: Water resources; maximum rainfall; surface runoff; Ly Son island

1 Introduction

In the world the water resources of islands in the sea and oceans are mostly derived from rainfall Therefore, we can see that the climate in general and rainfall regimes in particular have a very strong impact on the island's water resources Especially in the tropical monsoon regions, the rainfall in the rainy season is very large and in most months

of the year there is rain There have been many papers published research results on the relationship between rainfalls with the components of the water balance equation according

to different spatial scales Due to the small surface area of the island, the interaction between the island and the atmosphere is limited, although the impact of climate change and rainfall

on the water resources on the island is still very large, this has been described quite carefully

in works (Chapman 1985, Falkland 1991)

On the islands there may not be many hydrographic stations, but most have meteorological observation stations with fairly long data sets of several decades For that reason, the results of the calculation of climate parameters, frequency distribution as well as rainfall fluctuation trends have been published in a number of references (WMO 1966; Sen 1968; Ramsey 1989; Khaled et al 1998; Phan 1999; Yue and Wang 2004; Hennemuth 2013) The assessment results of climate and rain regimes provide the scientific basis for estimates

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of surface runoff and the total amount of water from rainfall (Sokolop and Chapman 1974;

Vu et al 2009; WMO 1986)

Results of water balance studies are often used for water planning and management, for example to allocate resources or implement new water projects Water balance studies can also be used to analyze surface water, groundwater or combined water resource systems Details of water balance equations and computations are contained in a number of references Water balance equations can be applied to small islands in two stages, namely, the surface water system and the groundwater system The reference zone for surface water system includes the atmosphere above the land and the surface itself and the soil moisture zone A surface water balance for a given reference can be expressed as follows:

Precipitation - evapotranspiration (from all sources) + surface water inflow - surface water outflow + groundwater inflow - groundwater outflow - increases in water storage

The main input term to the water balance equation of a small island is precipitation, generally in the form of rainfall Small island water resources are very sensitive to short term variations in rainfall due to their limited storage capacity within surface and groundwater systems Mean annual rainfall is a useful general guide to a particular islands but it is not useful for water balance studies on a very small island as knowledge of the temporal distribution over shorter time periods is required

That is reason why we put more attention to the typical durations of rainfall which would be applicable to very small islands The durations are often chosen as 1 hour, 3 hours,

6 hours, 12 hours and 24 hours (WMO 1981, 1994) In the present study, we have used rainfall record with a duration of 24 hours

To understand rainfall regime of Ly Son island, the authors have used statistical method applied for meteorological data analyses The analysis was carried out by using Mann - Kendall method, WMO publications (Wilks 2005)

Climate regime of Ly Son island is studied well The present study is on rainfall regime using the data set of the period 1985 - 2015 observed at the Ly Son Marine Meteorological Island Station

Ly Son island of Quang Ngai province is one of the major islands of Vietnam with the development of many activities and has potential to become a center of logistics Ly Son island is situated between 15022’00 and 15023’00N latitude and 109005’50 and 109008’20E longitude and 30 km far away from Vietnam mainland (Fig.1) Ly Son island has an area of 10.4km2 with the population of 19307 people (2016) Economic activities are mainly on fishery, garlic cultivation and tourism A total of drills and boreholes for water using are

2149 Increasing demands on water uses on the island has adversely contributed to the degradation and depletion of island water resources and especially salinity intrusion to the groundwater system (website: lyson.gov.vn, quangngai.gov.vn)

Ly Son island was formed by 5 eruptive volcanos Groundwater presents essential resource on Ly Son island, where the residents and tourists rely on aquifer for domestic supply from 2149 boreholes (2016) (Nguyen 2016; General Statistics Office of Quang Ngai,

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2016, http://lyson.gov.vn) As we know without a thorough understanding and knowledge

of the type and sustainability of island water resources, no informed decisions can be made about their policies of development, conservation and protection of the island Studies on water resources are policy - relevant in a context of rapid change and population growth, where groundwater resources can be expected to be increasingly relied upon in the next years

As many researchers have considered that water resources on Ly Son island must be focused on surface water resources with the original source from rainfall (Bui et al 2017) Rainwater provides an excellent freshwater supply especially in areas where surface water and groundwater are unavailable and very scarce or contaminated

Ly Son island has only one reservoir Thoi Loi - a supplied source only for agriculture

On the island there are many projects on groundwater exploitation There have been only a few studies on surface water resources on Ly Son island up to now

There is a Marine Meteorological Station on the island that was established in 1984 and came into operation in 1985 including rainfall recording for the period 1985 - 2015 The

Ly Son Marine Meteorological Island Station is one of the state stations on the system of marine meteorological stations of Vietnam That is a good condition with 30 years continuing recording data to study the variation of statistical parameters of rainfall and the surface runoff in turn

Figure 1 Ly Son island Figure 2 Ly Son Marine Meteorological Island Station

1.1 Characteristics of hydrology and water resources of Ly Son island

There is no river or stream on the islands During the rainy season there are some streams but it exists for a short time and endsalmost after the rain stops There is only one Thoi Loi water storage on the island which has been put in use since 2012 The Thoi Loi water storage has water volume of 27 x 104 m3 by design but there is only 60 - 70% of the

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water design volume in fact and this is the only one water resources supplying for agricultures of 60 ha of Ly Son island The water resources used for daily activities are mainly from drilling system and there are many problems due to salinity intrusions that happen on aquifer qh This situation requires drilling deeper to bazan aquifer (Bui et al 2014)

1.2 Oceanological characteristics

Semi diurnal irregular tide regime had dominated all the sea around Ly Son island The range of tide was 2.0m on the spring tide.The sea current had NE- direction with the speed

of 50 - 70 cm/s in the summer time and 30 - 60 cm/s in winter period with SW direction The average sea surface temperature was 26.10C, 2 - 30C lower than the main land The salinity changed from 30-31‰, the highghest value was observed at 34‰

1.3 Climate

The climate of Ly Son island is tropical monsoons and characterised by two seasons, wet and dry, persistent northeasterly and southwestely winds and occasional severe storms The average air temperature at sea level was 26.70C, maximum air temperature was 36.80C (August), minimum air temperature was 15.40C (March) From March through August is normally the driest season influenced by southwest wind The wettest season normally last from September through February of the next year being influenced by northeast wind

The average amount of hours of sunshine was about 2430.3 h/year Mean relative humidity was about 84.9% of all year around Mean evaporation was about 964.4 mm Mean wind speed was 1.5 mps and high value of wind speed 5 - 10 mps during the northeast wind season Maximal wind speed was 30 - 40 mps during typhoons Typhoons occur from September to November and the average number was 7 per year that strikes on Ly Son island (Bui et al 2017)

The main purpose of the paper is to access some findings on rainfall regime such as trend of rainfall, maximum values of daily, monthly and yearly rainfall From those results, the study comes to estimate surface runoff and the amount of surface water received from rainfall on Ly Son island These results may be useful to conclude that precipitation on Ly Son island is rather copious in the rain season and that is the reason why the study comes

to propose that on Ly Son island there should be preparation of all kinds of measures for collecting rainfall supplying for dry season

The paper is aimed to estimate a value of surface runoff for the entire small island Surface runoff for particular catchment can be measured using stream gauging techniques

or can be estimated When surface runoff records are unavailable or if records are missing for various reasons, surface runoff can be estimated by some calculated methods The most common techniques available are correlation with rainfall records Water balance equation

is convenient to analyze the water balance of small island for both surface water system and the groundwater system

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2 Methodology

2.1 Data collection

The Ly Son Marine Meteorological Island Station was established in 1984 and started collecting the time series data from 1985 to 2015 (Fig.2) Rainfall is measured by rain gauge which is read 4 times per day The set data is examined by experts at the Ly Son Marine

Meteorological Island Station

2.2 Frequency method and trend linear regression equation

In statistic computation of rainfall data there are used main formula such as:

Where R i isthe amount of precipitation in the period i; Rtb - average precipitation of the period computation

Standard deviation: (2)

Maximum of the data set: Rmax = MAX (R1, R2 …, RN) (3)

Minimum of the dataset: Rmin = MIN (R1, R2 …, RN) (4)

Amplitude of the data set: DRx = Rmax - Rmin (5)

Where: N is number of the data set following day, month, season and years

One of the methods analyzing tendency using in climate oscillation is regression method

Method of regression applying here comprised x variation by time t, x = f(t) f(t) is

function of linear or nonlinear We have regression equation for study on variation of rainfall

as follows:

(6) Where:

(8)

Where m x , m y are average values of x and y and S x , S y are standard deviation

r is regression coefficient and a- has shown variation of rainfall by time Sight of value

has shown tendency of increaseor decrease in precipitation

In this study, the Mann - Kendall test Z for evaluation of trends for sample size

greater than 30 was used For this case using a confidence level of 95%, a significant trend

will be confirmed if the value of Z is greater than 1.96 (Thorsten Pohlert, 2018)

2.3 Water balance equation and surface runoff

A simple commonly applicable surface water balance equation for a small island is :

Precipitation (P) - Evapotranspiration (ET) from all sources - surface runoff (SR) - groundwater recharge (R) - increases in soil water storage (AV) = W

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This equation can have terms deleted or added as appropriate It can be applied to a single catchment or to large area Sometimes a water balance equation is applied to an average situation, by using mean values of hydrological variables and assuming that they represent a long-term mean In this case, storage change terms are neglected Long term recharge values are better by averaging the results deduced from a long series of data

When the storage terms have been neglected in long term water balance, it is implied that mean values do not change As usual mean annual rainfall is a useful general application for study on hydrology of a particular island As common, small island water resources are very sensitive to short term variation in rainfall due to their limited storage capacity within surface and groundwater systems Monthly or weekly time series may be adequate for study

on water balance but generally a daily time series and sometimes an hourly time series is appropriate It is good to analyse rainfall and surface runoff and their relationships with maximum values (Yue et al 2002)

All of these concepts are useful to study surface runoff and regime of rainfall of the Ly Son island

We use surface water balance equation for islands as follows:

(9) Where:

P: Precipitation (mm)

ET: Evaporation (mm)

SR: Surface runoff (mm)

GWR: Ground water recharge (mm)

And surface water runoff equation for Ly Son island is as follows:

Base on results computed of surface runoff (SR), we can estimate potential surface

water for Ly Son island Amount of surface water runoff per year over an area is computed

by the formula as follows:

Where : Wyear (m3), Yyear: Surface runoff (mm), F lv: Area (km2)

Precipitation

The main input term to the water balance of an island is precipitation, generally in the form of rainfall Rainfall is measured by rain gauges at selected sites Rain gauges are commonly read daily but can also be continuously recorded

Evapotranspiration

The combined processes of evaporation and transmission are often referred as total evaporation Precipitation and evaporation are the two significant parameters of the water balance of small islands Brutsaert (1982) provides a more detailed and more recent account

of estimation methods for most practical applications of evapotranspiration Formula is based

on commonly available meteorological or evaporation data that are used to estimate

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evapotranspiration These formula use empirical values and functions derived from well-instrumented experimental sites

According to A Falkland (1991) values of hydrologic aridity index (P/E 0) for selected

small islands and climatic zone we can use high values of P/E 0 as humid climates in the Sulu Sea (west side of Philippines) for Ly Son island of Vietnam

Groundwater recharge from precipitation

Recharge from precipitation constitutes all or most of the recharge input to groundwater on small islands In humid tropical areas, mean recharge may attain hundreds

of mm per year or 25 to 50% of precipitation if it is not rejected as surface runoff when either the water table is close to ground level or infiltration capacity of the soil is low Recharge values of less than 10% or even less than one percent of rainfall are common A number of methods are outlined for the calculation or estimation of recharge (Falkland 1991; Nguyen 2016)

3 Results

3.1 Computed maximal rainfall of Ly Son island

Using daily rainfall data to assess and analyse rainfall intensity Table 1 has shown results of computing standard deviation and variation of monthly rainfall on Ly Son island The months have been chosen represented for seasons, January for winter, July for summer, and April and October for transition time Through the table, we can see the standard deviation of rainfall in summer and winter in range of 81 - 100mm The months have the highest values of standard deviation inrange of 200 - 350mm, being coincident with months with the highest rainfall (September to October) While the standard deviation of yearly rainfall is rather high and it approximate to be 731 mm (equivalent to nearly 13% of variation)

So the monthly rainfall data is a very good pattern

Figure 3 shows the maximum and monthly average rainfall on Ly Son island in the whole period of 1985 - 2015 We can see that the maximum rainfall was approximately 1900

mm in September This is the month of rainy season and with such large rainfall numbers, the addition of surface water or underground water is worth to investigate The analysis has showed that the maximum daily rainfall in Ly Son was moderated, with the highest value of 418.4 mm occured on 18 May 1986 (Fig.4) October has a maximum rainfall with average value

of 519.3 mm and maximum value of 970.7 mm (Table 6) The mean annual rainfall on Ly Son island recorded over 30 years (1985 - 2015) was 2278.8 mm (Table 3) Over the period of record the rain range was from 1208 mm to 3238 mm (maximum of 3238 mm occurring in 2009 and

a minimum of 1208 mm occurring in 2004) The average number of rainy days was 124 days per year and maximum was 160 days (1996) Rainfall can occur in all months of the year The dry season last from March until August, meanwhile the rainy season from September until February next year with the most rain events occurring in the rainy season (78.1%)

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Table 1 Standard deviation (S) and variation (Sr) of rainfall data (1985-2015) of Ly Son island

Figure 3 Maximum and monthly average

rainfall of Ly Son (1985-2015)

Figure 4 Maximum daily rainfall in months

of Ly Son (1985-2015)

Table 2 Maximum daily rainfall frequency (%) in Ly Son in various periods (1985-2015)

Periods

Ranges of rain (mm)

Figure 5 Number of days of monthly rainfall in

Ly Son with range of 50-100 mm

Figure 6 Number of days of monthly rainfall in Ly Son with range > 100 mm

The number of days with rainfall in the range of 50 - 100 mm was highest in October, about 6 days and the number of days of monthly rainfall with range >100 mm reached the peak of 5 days in September (Fig.5, Fig.6) Most of daily rainfall in the study area was in range of less than 50mm, accounting for 57.8% And the rainfall in range of 50 -

100 mm with frequency of 30.96% and the rainfall in range of more than 300 mm with frequency of 2.6% for the whole period 1985 - 2015 (Table 2) The maximum daily rainfall values changed with each period; from 1985 to 1990, this value dropped sharply, down by 6mm/year but in the next cycle (1991 - 2000), there was a slight increase and in 2001 - 2015 period, the maximum daily rainfall went up dramatically (Fig.7) But in summary, the trend

of maximum daily rainfall was slightly reduced in the whole period of 1985 - 2015 (Fig.8) These signs may be associated with El Nino activity in the West Pacific

Figure 7 Rate of maximum daily rainfall on

Ly Son island

Figure 8 The trend of maximum daily rainfall on Ly Son island

3.2 The trend of average rainfall and maximum rainfall in Ly Son island

Trend of average rainfall

Using total monthly rainfall, annual rainfall and maximum daily rainfall to analyse the trend (Pohlert, 2018) Data are broken down into four phases: before 1991, 1991 - 2000,

2001 - 2015 and the whole period from 1985 to 2015 In each period, the rainfall has changed significantly The average rainfall observed from 1985 to 1990 was about 2363 mm which is higher than the average rainfall of 1985 - 2015 period (2278 8 mm) Average rainfall in 1991

- 2000 period was lower than in previous years but was generally greater than the annual years, about 61.2 mm In the recent period (2001 - 2015), rainfall has tended to decrease by about 100 - 150 mm compared to the previous two periods, corresponding to a reduction of over 70 mm compared to the all-year average (Fig.9)

Figure 9 Monthly average rainfall in Ly Son in each period

The trend of rainfall change in Ly Son with several representative months has been shown in Figures 10 and 11 The results show that the monthly rainfall in Ly Son has a high rate of change and there is no clear rule In the first month of dry season (January), the rainfall is likely to increase, while in the first month of rainy season (July), the total rainfall decreases sharply about 3 - 5 mm /year (Fig.10 and Fig.11)

Figure 10 Trend of Ly Son rainfall in

January (1985 - 2015)

Figure 11 Trend of Ly Son rainfall in July

(1985 - 2015)

Figure 12 Trend of rainfall in Ly Son in

the period 1991 - 2000

Figure 13 Trend of rainfall in Ly Son in the

period 1985 - 2015