## 1 Introduction  
Water-level monitoring has been widely used to reduce the danger of flooding disasters, ensuring the safety of ship channels and monitoring the aquatic environment. Field monitoring by people is still used, but sensor monitoring is being adopted by more monitoring stations. Currently some commercial product-based monitoring systems can be found in China, like the JWR-SJ water-level monitor from the Beijing Jiawer Co. [1] or the HG35-QSW water-level monitoring system of the Beijing ZhongxiCo. [2]. Others are water-level monitoring systems based on wireless sensor networks such as the DATA86 water-level monitoring system of Tangshanpingsheng Co. [3]. Similar monitoring products are also found overseas, such as TD-DIVER, an underground monitor by Holland Foreign Talent and YSI Level Scout, a water level monitor with large data storage capacity from Doppler U.S. However, these on-the-spot automated devices still need people to download the data in the field later, therefore, researchers have sought to design and implement remote water environmental monitoring systems. Some are based on wireless sensor networks through the transmission of sound waves [4]. Others are based on an _ad hoc_ network of wireless nodes [5-7], or adopt handheld data gathering and return data to the server using a GPRS network. Due to the interference from the transmission distances among the wireless nodes of the sensor network, such monitoring systems are more applicable to small-scale water monitoring scenarios. Considering the vast territory and complicated hydrological environments of China, the applicability of the above methods is greatly limited.  
Let us consider, for instance, Poyanghu Lake, located in the Hengshan shoals of China, where seasonal flooding occurs. A practical method is also needed to simultaneously monitor multiple ground water wells which distribute water. As a third example, many quake lakes were formed during the Wenchuan Earthquake in Sichuan Province of China on May 12 th, 2008. The water-level of those quake lakes increased rapidly and threatened life and property, so there was an urgent need to monitor the water-levels quickly and accurately, but the appropriate Water Department still used field monitoring by people, which is very inefficient. In addition, the safety of the monitoring personnel was threatened by the aftershocks, landslides, and rainfalls, so it was hard to accomplish the water-level measurements. Under such circumstances, a remote water-level monitoring system could provide synchronized data directly to the concerned Water Department and implement real-time monitoring and provide early warning of the water-levels of quake lakes.  
The remote wireless water-level monitoring system adopts the self-developed embedded ZKOS operating system in order to ensure an expansible and real-time system and also reduce the cost. The system timer is revised or adjusted according to the satellites synchronized through GPS chips, which will guarantee the synchronicity among the systems located in different regions. Considering most of the sites are in the field, a program is designed and developed so that the system can be updated remotely. The related software is also developed in the remote server to monitor and control the frequency of the system. Therefore, this system can be customized according to users' inquiries. Currently, this system is being used to monitor the water-levels of Poyanghu Lake. It is also planned to use it to continuously monitor the degree of mineralization of groundwater and water-levels in Quzhou, Hebei Province, China.