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

Rainfall Regime and Its Impact on Water Resources on Ly Son Island, Central Vietnam Bui Xuan Thong(1)(*), Nguyen Van Dan2, Nguyen Ngoc Ha3, Van Phu Hung1 (1) Institute of Oceanography and Environment, Hanoi, Vietnam Institute of Resources and Environmental Water, Hanoi, Vietnam (3) National Center for Water Resources Planning and Investigation, Hanoi, Vietnam * Correspondence: buixuanthonghunre@gmail.com (2) 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 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 141 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 hour, hours, 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 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, 142 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 Ly Son island Figure 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 143 water design volume in fact and this is the only one water resources supplying for agricultures of 60 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, - 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 - 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 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 144 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 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: Deviation: (1) Where Ri 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 - (5) Rmin 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: (7) (8) Where mx, my are average values of x and y and Sx, Sy 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 145 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 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: (10) 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: (11) Where : Wyear (m3), Yyear: Surface runoff (mm), Flv: 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 146 evapotranspiration These formula use empirical values and functions derived from wellinstrumented experimental sites According to A Falkland (1991) values of hydrologic aridity index (P/E0) for selected small islands and climatic zone we can use high values of P/E0 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) Results 3.1 Computed maximal rainfall of Ly Son island Using daily rainfall data to assess and analyse rainfall intensity Table 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 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%) 147 Table Standard deviation (S) and variation (Sr) of rainfall data (1985-2015) of Ly Son island Parameters Month represented Yearly January April July October S (mm) 100 102 81 197 731 Sr (%) 79 163 173 38 13 Figure Maximum and monthly average rainfall Figure of Ly Son (1985-2015) Maximum daily rainfall in months of Ly Son (1985-2015) Table Maximum daily rainfall frequency (%) in Ly Son in various periods (1985-2015) Ranges of rain (mm) Periods =300 1985-1990 51.2 26.2 10.7 8.3 3.6 1991-2000 58.3 18.3 15.8 5.8 1.7 2001-2015 61.1 19.4 12.5 4.2 2.8 1985-2015 57.8 20.7 13.2 5.7 2.6 EDESUS2019, FOR PEER REVIEW https://edesus.ueb.edu.vn Figure Number of days of monthly rainfall in Ly Son with range of 50-100 mm Figure 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 days and the number of days of monthly rainfall with range >100 mm reached the peak of 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 Rate of maximum daily rainfall on Figure The trend of maximum daily Ly Son island 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 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 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 - mm /year (Fig.10 and Fig.11) Figure 10 Trend of Ly Son rainfall in January (1985 - 2015) Figure 12 Trend of rainfall in Ly Son the period 1991 - 2000 Figure 11 Trend of Ly Son rainfall in July (1985 - 2015) in Figure 13 Trend of rainfall in Ly Son in the period 1985 - 2015 Total rainfall has increased in all periods The highest increase was in the period 1991 - 2000 with an increase of 79.9 mm per year (Fig.12) For 30 years from 1985 to 2015, rainfall has increased but not significantly, about 0.1 mm / year (Fig.13) Trend of maximum daily rainfall Concerning the maximum rainfall in a day, the calculated results show that there is no significant change in frequency over time According to the distribution of rainfall, the majority of daytime rainfall in range of less than 50 mm had frequency of more than 50% in all periods, the frequency of moderate rainfall in range of 50 - 300 mm was between 4.2 26.2% Particularly heavy rainfall in 24 hours (≥300 mm) accounts for a small proportion, ranging from 1.7 - 3.6% and a full-time average of 2.6% (Table 2) While the total rainfall in all periods has increased, the maximum rainfall in the period 1985 - 1990 and the whole 1985 - 2015 period are likely to decrease However, for the recent years, from 2001 to 2015, the total annual rainfall as well as the maximum daily rainfall is likely to increase (Figures 12 and 13) The average annual rainfall measured on Ly Son island was approximately 2278.8 mm while evapotranspiration was 877.2 mm (Table 3) In the rainy season, from September to February, rainfall reached 1779.6 mm, accounting for the vast majority of the average rainfall throughout the period (Table 4) Reserve this abundant rainfall in rainy season will solve the lack of water for living and economic development on the island Table Average values of rainfall and evapotransporation of Ly Son meteorological stations (19852015) Jan Month Fe M Ap Ma Ju b ar r y n Jul Au Sep Oct No g Dec v Total Annu al Parameters Precipitation 121 45 88 59 114 57 (mm) Evapotranspir 64.4 49 45 48 68.2 90 7 ation (mm) 50.2 129 424 519 413 255 2278 8 103 104 81.5 74.4 73.1 73.2 877.2 Table Seasonal rainfall distribution Stati Ann on ual Precipitation in dry months Rainfall in rainy months M A Ma Ju Ju Au Dry Se Oc No De ar pr y n l g seas p t v c Jan Fe Rai b ny on seas on Ly m 2278 88 59 11 57 50 12 499 42 51 41 25 12 45 177 Son m 4.2 9.3 4.8 9.3 3.2 5.5 1.2 9.6 % 100 5.0 2 5.7 21.9 18 22 18 11 5.3 78.1 % 6 3.3 Calculating the total surface runoff Based on formula (9), (10) and (11) described above the surface runoff components are computed With the annual average rainfall of 2278.8 mm/year and the area of Ly Son island of 10.4 km2, the total amount of rainfall water on the island was 22.742 x 106 m3 /year or 62308 m3 /day Table Results of annual average surface runoff calculation for 1985 - 2015 period Mo Average monthly Evaporatio GWR (Ground GWR (Ground SR (surface nth rainfall (P) (mm) n (E) (mm) water recharge) water recharge) runoff) (mm) (%) (mm) Jan 121.2 64.40 6.45 50.31 Fe 45.7 49.70 2.98 - 88.2 45.20 5.27 37.75 59.3 48.70 4,40 6.22 114.2 68.20 6,22 39.81 Jun 57.9 90.20 4,12 - Jul 50.2 103.90 3.43 - Au 129.3 104.70 7.29 17.28 424.8 81.50 10,23 333.12 Oct 519.3 74.40 11.01 433.91 No 413.2 73.10 10.34 329.72 255.5 73.20 10.45 171.82 2278.8 877.20 116.58 1419.94 b Ma r Ap r Ma y g Se p v De c Tot al (-) is the value that does not generate surface flow in that month According to the calculation results in the table, the average surface water runoff for period 1985 - 2015 was 1419.94 mm rounded to 1420 mm With an area of Ly Son island of 10.4 km2, the total annual surface water flow on the island is 14.17 x106 m3/year It is noticed that the use demand of water of Ly Son island was 17597 m3/day (for domestic use is 1051 m3, for agriculture was 16246 m3 and for other was 300 m3) and for the whole year, Ly Son island needed 6.422 x 106 m3 The computing results are also shown that if there are good measures collecting all kinds of rainfall, there will be enough fresh water supplied for the whole island in a year Table The computing results of surface runoff (SR) by maximum rainfall in the period of 1985 2015 Maximum Rainfall Rainfall Evaporation GWR GWR SR W (P) (E) (mm) (%) (mm) (mm) (x10 (mm) % Average surface m) flow (x106 m3) Maximum rainy 418.4 68.2 0.07 29.29 320.91 3.2 22.58 970.7 74.40 0.02 20.58 875.72 8.74 61.67 519.3 74.40 0.02 11.01 433.91 4.33 30.55 14.17 days (May18,1986) Maximum Rainfall of October,1998 Average rainfall for October (1985 2015) The computing results of surface water runoff and water resources originating from rainfall (Table and Table 6) have shown the potential yield of rainfall on Ly Son island One day with maximum rainfall of 418.4 mm caused a total surface water on the whole island 3.2 x 106 m3 occupied 22.58% of average annual surface water One month (October 1998) with maximum rainfall of 970.7 mm greatly contributed to average surface flow up to 61.67% and average rainfall of October caused 4.33 x 106 m3 of surface water flow occupied 30.55% of annual surface flow on the island Such figures have shown that if there are good solutions chosen for keeping all the water flows, water shortage on the island will be improved However, in practice, solutions to store rain water by boreholes are limited and almost of the boreholes are salty at present time Furthermore, on Ly Son island, there is only one Thoi Loi reservoir supplying water for agriculture Therefore, it is necessary to have the solutions of works, non-works, planning and use of water resources as well as the government concern to soon overcome the water scarcity on Ly Son island Conclusions and discussion 4.1 Conclusion The results of present study show that the rainfall amount of all periods has increased tendency with the value of 0.1 mm/year The mean annual precipitation for the period of 1985 - 2015 was 2278.8 mm and rainfall is distributed throughout the year In the dry months, the average rainfall was 499.2 mm while in the rainy months the rainfall was 1779.6 mm In addition, it should be noticed that the evapotranspiration in the months had maximum rainfall oscillates in range of 2- 4% Prevalent rainfall in range of less than 50mm had frequency of 57.8%, the rainfall in range of 50 - 100 mm occupied 20.7% and the rainfall in range of more than 100 mm had frequency of 21.5% Maximum daily rainfall was 418.4 mm (occurring on 18 May, 1986), maximal value of month with maximum rainfall was 970.7 mm (October) while the average of maximal rainfall value of month with maximum rainfall was 519.3 mm Based on water balance equations and results of computing of maximum rainfall as shown above, this study has pointed out that the average surface runoff for period 1985- 2015 was 1420 mm corresponding to mean annual precipitation of 2278.8 mm With the area of Ly Son island of 10.4 km2 the estimated potential yield of surface water by whole island was 14.17 x 106 m3 /year Furthermore, this study has also shown that the surface runoff of the day with maximum rainfall (418.4 mm, 18 May 1986) was 320.91 mm and amount of water flow on the island was 3.2x106 m3 occupied 22.58% of all year/the whole year The surface runoff of the month had maximum rainfall (October 970.7 mm) was 875.72 mm and estimated potential yield of rainfall by whole island is 8.74 x 106 m3 occupying 61.67% of all year/the whole year Surface runoff of the average of month had maximum rainfall (October 1998 519.3 mm) was 433.91 mm and the estimated potential yield of surface runoff by whole island is 4.33 x 106 m3 occupying 30.55% of all year The results of the study on rain regime and surface runoff on Ly Son island will be the basis for further implementation of other problems simulating the interaction between rainfall and groundwater replenishment for aquifers of Ly Son island On the other hand, the data on the maximum rainfall as well as the intensity of rainfall on Ly Son island have important reference values to address the problem of exploiting the rain water potential in the island 4.2 Discussion Precipitation is the most important element of water resources on island Thus, we need concentrating on researches about rainfall regime From that, calculating surface runoff then creating basis for research underground water recharge from rainfall for the next study Time series data of rainfall collection in 30 years on Ly Son island were high reliability and also applied on water resources research and water resources management in general The statistical values about rainfall such as daily maximum rainfall, monthly maximum rainfall also have standard deviation, variation within the allowable limits (Table 1) The tendency of precipitation increased in almost periods of observation (Fig.10, Fig.11, Fig.12, and Fig.13) In the present study, it is not considered an abnormal rainfall from storm surges that pass through Ly Son island According to the year variable, rainfall in Ly Son is unevenly distributed between months of the year and does not show rainy season or dry season The difference here is only the rainy season and less rainy season In the year in which two extremes of rainfall occur, the first extremes coincide with the peak of the wet season (from September to November) and the second occurs most often in May or June According to the intensity of rainfall, the frequency of maximum rainfall in the day is common at the moderate rainfall level With heavy rain, Ly Son has a frequency ranging from 19.5 - 23.0% Prior to the year of 2000, the rainfall in the study area was very high during the rainy season However, in the recent period (2001 - 2015), very high rainfall during the day sometimes occurs in low rainfall months (from January to March) Rainy days, heavy rains tend to decrease, but the number of days with heavy rains tends to increase There are no obvious and variable changes in the trend of monthly, seasonal or yearly rainfall Total rainfall has increased in all periods, but peak rainfall throughout the period tends to decrease Unusual changes in precipitation, rainfall intensity as well as large differences between rainy season and dry season on Ly Son island are related to the anomalies of climate change in the Central Vietnam Therefore, there is a need for more thorough studies on the impact of climate change on rainfall regime on Ly Son island The method of assessing the surface water potential from rain water only if using the equation of water balance is not enough, but it must consider the surface runoff distribution on the whole island, so to use other models of simulation (Bui et al 2019) In the equation of surface water balance some parameters still have to accept the unobserved values at the island The Soil Water Assessment Tool (SWAT) combined with Geographic Information System (GIS) should be recommended for the study of water resources in Ly Son island Acknowledgments This study is a part of the National Research Project granted by Ministry of Science and Technology of Vietnam (The Project code: KC 09 04/16 - 20 namely “Estimation of potential and variation of surface, underground water resources and proposed appropriate measures for water resource management to support economics - social development on some important islands”) The authors would like to thank Ministry of Science and Technology of Vietnam for all help References Bui, V.D., Stattegger, K., Nguyen, T.T., Phung, V.P., Tran, T.D., Bui, X.T (2014) Late Pleistocene - Holocene seismic stratigraphy of Nha Trang shelf Marine Petroleum Geology (2014), 1-12 Bui, X.T., Le, T.D., Truong, V.C (2017) Estimation of Surface Water in Ly Son island, Journal of Marine Science and Techonology, (T.17) Vietnam Academy of Science and Technology, ISSN 1859 - 3079, 353-363 (In Vietnamese) Bui, X.T., Nguyen, V.D., Nguyen, M.T, Nguyen, N.H (2019) Using the method of Soil Conservation Service Curve Number (SCS CN) combined with the Geographic Infomation System (GIS) to estimate the surface runoff on the Co To island, North Vietnam International Journal of Advance Engineering Research and Science 6(9), 59-75 Chapman T.G (1985) The use water balance for water resources estimation with special reference to small islands Australian Development Assistance Bureau Bull 4, 39 Falkland, A (1991) Hydrology and Water Resources of small island: A practical guide United Nations Educational, Scientific and Cultura Organization (UNESCO) Hennemuth, B., Bender, S., Billow, K., Preier, N., Keup, E., Krager, 0., Mudersback, C., Radermacher, C., Schetter, R (2013) Statistical methods for the analysis of simulated and observed climate data, applied in projects and institutions dealing with climate change impact and adaptation CSC Report 13, 41- 77, Climate Service Center, Germany Khaled, H.H., Ramachandra, R (1998) A modified Mann -Kendall trend test autocorrelated data, Hydrology, 204(1-4), 182 - 196 Nguyen, V.D (2016) Ground water in Ly Son island and exploitation oriented Natural Resources and Environment Journal 24 (254), 12 (In Vietnamese) Phan, V.T (1999) Statistical Method in Climate Hanoi National University Publishing, 41-77 ( In Vietnamese) Pohlert, T (2018) Non - Parametric Trend Tests and Change point Detection 1-18p This work is licensed under a Creative Commons License (CC BY - ND 4.0 http://creativecommons.org/licences/by-nd/4.0 for details Quang Ngai Statistic Year Book, 2016 General Statistics Office of Quang Ngai Pubishing, 377 pages (In Vietnamese) Ramsey, P.H (1989) Critical values for Spearman’s rank order correlation J Educ Stat 14, 245-253 Sen, P.K (1968) Estimates of the regression coefficient based on Kendall’s Tau American Statistical Association, 63(324), 1379-1389 Sokolop, A.A., Chapman, T.G (1974) Methods for water balance computation UNESCO, Studies and Rport in Hydrology, N 17 Paris, 127 Vu, T.H, Chu, T.H, Phan, V.T (2009) Trend of maximume day rainfall in VietNam in the period of 1961 -2007 Journal of Nature Science and Technology Hanoi National University, Number 25, 3S Website of Ly Son District: Ly Son http:lyson.gov.vn Website of Quang Ngai Province: www.quangngai.gov.vn Wilks D.S (2005): Statistical Methods in the atmospgeric sciences, Department of Earth and Atmospheric siciences, Cornel University 26 - 27, 35- 39, 104- 109 WMO (World Meteorological Organization) (1966) Some Methods in Climatological Analysis (WMO/TN No 81, WMO-No 199) WMO (World Meteorological Organization) (1981) Selection of Distribution Types for Extremes of Precipitation (OHR-No 15, WMO/TN-No 560) WMO (World Meteorological Organization) (1986) Manual for Estimation of Probable Maximum Precipitation (OHR-No 1, WMO/TNNo 332) WMO (World Meteorological Organization) (1994) Guide to the Applications of Marine Climatology (WMO-No 781) Yue, S., Pilon, P., Phinney, B., Cavadias, G (2002) The influence of autocorrelation on the ability to detect trend in hydrological series Hydro Processes 16, 1807-1829 Yue, S., Wang, C (2004) The Mann-Kendall test modified by effective sample size to detect trend in serially correlated hydrological series Water Resource Manage 18, 201-218  ... maximum daily rainfall on Figure The trend of maximum daily Ly Son island rainfall on Ly Son island 3.2 The trend of average rainfall and maximum rainfall in Ly Son island Trend of average rainfall. .. Son island Using daily rainfall data to assess and analyse rainfall intensity Table has shown results of computing standard deviation and variation of monthly rainfall on Ly Son island The months... solutions of works, non-works, planning and use of water resources as well as the government concern to soon overcome the water scarcity on Ly Son island Conclusions and discussion 4.1 Conclusion