Design of an automatic hydro-meteorological observation network for a real-time flood warning system: a case study of Vu Gia-Thu Bon river basin, Vietnam

This paper presents an interdisciplinary approach, along with Vietnam ’ s legal frameworks, to design an automatic hydro-meteorological (HM) observation network for a real-time ﬂ ood warning system in Vu Gia-Thu Bon (VGTB) river basin, Vietnam. The automatic HM monitoring network consists of weather-proof enclosures containing data loggers, rechargeable batteries, sensors for air temperature, air humidity, solar radiation, wind speed, water level with attached solar panels and mounted upon masts located at ﬁ xed ground stations. A total of 20 meteorological stations and ﬁ ve hydrological stations have been built in VGTB river basin. To capture changes in weather and stream ﬂ ow in the basin, the 5-minute and half-hour recording frequency options were set for meteorological and hydrological variables, respectively. All HM data was transmitted every 30 minutes to the data server at the data processing centre via Global System for Mobile Communications (GSM)/General Packet Radio Service (GPRS) network. These data were then input into hydrological-hydraulic models for inundation simulation in the basin. The results showed that the performance of ﬂ ood simulation at hourly time step has signi ﬁ cantly improved during ﬂ ood events in September and November 2015. Overall, near-real-time HM data recording from an automatic monitoring network proved bene ﬁ cial for an ﬂ ood early warning system.


INTRODUCTION
HM variables such as rainfall, air temperature, air humidity, solar radiation, wind speed, water discharge, and water level are the key inputs in flood forecasting and warning. In the absence of rain observation stations, significant errors occurred in simulating flood peaks in a semi-arid basin (Jene ). Theoretical reliable rainfall-runoff models will not accurately predict if rainfall input data is incomplete or inaccurate (Keith ). A dense network of rain gauge stations gave better results of overflow forecast than sparse rainfall station density (Gregory et al. ).
In theory, to collect HM data on a specific region, it is necessary to design an extensive monitoring network to fully and accurately cover the spatial heterogeneity of hydro-climate parameters on that area. However, when it comes to large basins or basins with high hydro-climatic variability, it is, in fact, unfeasible to install HM monitoring stations at all locations. To solve this problem, optimization approaches have been proposed to determine the minimum number of stations in the most appropriate locations while ensuring maximum collection of HM data. Coefficient of variation, key station network, spatial correlation and entropy were used to determine the number of additional rain gauge stations and the number of redundant stations (Panigrahy & Mani ). The optimal location of new monitoring stations in an existing rain monitoring network was evaluated using a geostatistical method and Geographic Information System (GIS) (Barca et al. ). Kriging was combined with entropy to determine the optimal number of rain gauge stations and spatial distribution of these stations This study aims to introduce an automatic HM observation network optimally designed for a real-time flood warning system in VGTB river basin, a large river system in the Central Coast of Vietnam. First, the flood regime of VGTB river basin is analysed to provide a conceptual HM observation network framework for this basin. Second, we use an interdisciplinary approach along with Vietnam's legal frameworks to optimally design an automatic HM observation network for a real-time flood warning system in VGTB river basin. Finally, we discuss advantages and disadvantages of the established HM observation network.

Study area
The VGTB river basin is located in Central Vietnam with an   End users are categorized into citizens and government officers, each having different access rights to the flood warning system database. Being a citizen account, the user can only view, query HM and flood data in the form of maps and charts and receive flood warning messages dependent on the specific situation. Meanwhile, as a government officer, the user not only owns the above functions, but also has the right to use advanced functions such as aggregating, exporting HM data, managing accounts and sending flood alert messages via SMS.

Design of automatic meteorological observation network
The process of optimising the number and location of weather stations on the VGTB river basin includes two phases: office work and field work (see Figure 4). The office work aims to allocate the optimal number of weather stations for each meteorological zone, and determine suitable areas for installing weather stations. The field work aims at examining field conditions (terrain, obstructions, and households), distances between stations and distances from weather stations to base transceiver stations to select the optimal location for installing weather stations that meet Vietnam's legal frameworks.
Based on the input data including geographic location and rainfall of rain gauges/HM stations, road networks, river networks and land use, the office work was carried out sequentially according to the following five steps.
Step 1: Based on the rainfall measurement data at rain gauges, HM stations in VGTB river basin, we assessed the accuracy of the existing weather network by coefficient of variation.
Step 2: Based on the coefficient of variation determined in the previous step, we calculated the optimal number of weather stations to be installed on the basin in order to meet the requirements of a flood warning system.
Step 3: From the rainfall measurement data and geographic location of rain gauges/ HM stations, we conducted meteorological zoning in the basin.
Step 4: From the optimal number of weather stations (Step 2) and the meteorological zones (Step 3), we allocated weather stations for each meteorological zone.
Step 5: Based on road networks, river networks and land use data, we overlaid these data to determine suitable areas for installing weather stations according to Vietnam's legal frameworks.
Design of an automatic hydrological observation network Similar to meteorological networks design, the process of optimizing the number and location of hydrological stations on VGTB river basin also involves two phases: office work and field work (see Figure 5). The office work aims to determine the optimal number of hydrological stations, and find suitable river sections for installing these stations. The field work aims at examining field conditions (terrain, road accessibility and households), and distances from hydrological stations to base transceiver stations to select the optimal location for installing hydrological stations that meet Vietnam's legal frameworks.
Based on the input data including terrain, satellite images and river networks, the office work was carried out sequentially according to the following three steps.
Step 1: Based on topographic map, we classified the topography of VGTB river basin into two types: hills/plains and mountains.
Step 2: Based on WMO regulations on the minimum density for stream gauges by terrain, we determined the optimal number of hydrological stations.
Step 3: Based on satellite images and river networks, we identified suitable river sections for installing hydrological stations meeting two groups of criteria according to WMO regulations and Vietnam's legal frameworks. Moreover, the location of the hydrological station must support the calibration and validation of SWAT model for upstream streamflow and HEC-RAS model for the downstream water level.

RESULTS AND DISCUSSION
Automatic meteorological observation network

Optimal number of weather stations
Currently, there are two rain gauge systems operating independently on VGTB basin: one includes rain gauges located upstream of large reservoirs under the management of the (2) river bed is relatively stable, water surface is without sudden changes (expansion or contraction); (3) near road networks for convenient construction; (4) construction areas must be at least 4 m 2 , flat (with a slope of less than 15 ) and have a stable ground structure; (5) construction areas must be covered with mobile coverage to ensure signal transmission.
reservoir owners, the other includes rain gauges and hydrometeorological stations installed throughout the basin managed by the Mid-Central Hydro-Meteorological Centre.
However, due to the privacy of the reservoir owners, we cannot access their precipitation data. Therefore, only rainfall data provided by the Mid-Central Hydro-Meteorological Centre was used. According to the Mid-Central Hydro-Meteorological Centre, there are 13 rain gauges and HM stations in operation on VGTB river basin (see Figure 6). Among them, nine stations have complete and valid data for optimal analysis of HM stations. The multi-annual average rainfall from 1980 to 2013 of these stations is shown in Table 1. Based on rainfall measurement data at rain gauges and HM stations, we where p is the current error in estimating rainfall over the basin, C v is the coefficient of variation of rainfall and N is the number of existing weather stations in the basin. The results showed that the error of the existing weather network was about 7.47%.
Based on C V determined in the previous step, we calculated the optimal number of weather stations to be installed on the basin with an error of 5%. To meet the requirements of the flood warning system, the accuracy of the weather network must be at least 95%. As a result, the required number of weather stations should be 20.
Spatial interpolation of multi-annual average rainfall in the study area was based on rainfall data series in the trips that took place during December 3-5, 2014, identified 20 optimal locations for installing weather stations that met the above criteria (see Figure 7).

Components of an automatic weather station
The automatic weather station consists of a weather-proof enclosure containing data logger, rechargeable battery and meteorological sensors (rain gauge, thermometer, hygrometer, pyranometer and anemometer) with an attached solar panel and mounted upon a mast (see Figure 8). To capture changes in weather in the basin, the 5-minute recording frequency option was set. All meteorological data were transmitted every 30 minutes to the data server of the data processing centre via the GSM/GPRS network.

Automatic hydrological observation network
According to  ) on the minimum density for stream gauges by terrain, it was necessary to install at least four stations for hilly/plain areas and one station for mountainous areas. Thus, the total number of hydrological stations that need to be installed on VGTB river basin was five.
Based on satellite images and river networks, we ident-

CONCLUSIONS
Although VGTB river basin is ranked number eight among the largest river basins in Vietnam, with an area of over 10,000 km 2 , there are currently only four rain gauges, one

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.