Agilent Measurement Journal - Issue 3 - 2007 - (Page 71)

Heat can also be used as a tracer. Because the temperatures of streams and subsurface springs differ, temperature measurements can provide a distributed, real-time look at the interaction of stream waters and their surrounding groundwater aquifers. Examining stream dynamics with the DTS New distributed temperature-sensing applications in hydrology are being developed at Oregon State University. Examples include measuring ﬂow patterns in lakes via temperature, upwelling of water-borne pollutants in abandoned mine shafts, and the thermal interactions of air and snow.6 Figure 2. An IP66/NEMA4 enclosure prevents moisture from interfering with instrument operation. In remote deployments, power consumption is a critical factor. The DTS offers a nominal power consumption of 15 W (< 40 W peak) and can use DC sources such as batteries and solar panels. Measuring the water cycle The water cycle begins with the sun heating the oceans and lifting moisture to the clouds. Precipitation falls to the Earth and starts its journey back toward the sea, driven by gravity. Water on the Earth’s surface is easily seen in rainwater runoff to lakes, rivers and streams. Less visible is the water below the ground. Groundwater makes up 98 percent of available fresh water and its importance can not be underestimated: It supplies 40 percent of the fresh water in the United States and 70 percent in China.3, 4 In addition to human uses, groundwater is essential as a freshwater source for springs, rivers, lakes and their surrounding habitats. Despite its critical role in the water cycle, the interaction of surface water and groundwater is difﬁcult to measure and therefore difﬁcult to understand, model and manage. Getting a local look at the water cycle typically involves the use of hydrologic tracers. A variety of passive (O16/O18, tritium, CFCs) and active (dyes) tracers allow hydrologists to piece together a detailed picture of the path taken by water both above and below the ground. For instance, measurements of O16/O18 ratios allow the determination of “residence time,” which is the average time the water spends in a given reservoir. These measurements often provide the key data needed to determine the origin and subsequent path water takes during its extended journey underground.5 For temperature-based measurements of ﬂow patterns, environmentally rugged — but otherwise standard — communications optical ﬁbers are placed in streams as shown in Figure 3. The distributed temperature sensor locates groundwater sources by looking for steps in the temperature change indicative of groundwater inﬂux. Several different methods, all based on conservation of energy and mass, enable not only the determination of the location of groundwater inputs but also quantitative estimates of the groundwater inputs to streamﬂow. One such method starts with measurements of temperatures upstream and downstream from the groundwater source. Coupling this information with knowledge of the groundwater temperature enables estimates of changes in the ﬂow rate using the following relation: Q0 = Qi () Tg –Ti Tg –To In this equation, Qo is the streamﬂow after the groundwater source (usually reported in cubic feet per second), Qi is the streamﬂow before the groundwater source, Tg is the groundwater temperature, Ti is the temperature before the groundwater source, and To is the temperature after the groundwater source.7 Agilent Measurement Journal 71

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 3 - 2007

Applying Ingenuity to the Measurement of Emerging Technologies Contents Emerging Innovations Department Delivering Bigger Benefits by Optimizing Customer Workflows Measuring Material Properties at the Nanoscale Utilizing In Situ Atomic Force Microscopy in Life Science, Pharmaceutical and other Bio-Related Applications WiMAX™: Plotting a New Path to Global Mobility Addressing the Triple Complexity of Triple-Play Networks What Next for Mobile Telephony? Exploring the Inner Workings of Tire-Pressure Monitoring Systems Developing, Assessing and Applying a High-Resolution Thin-Film Magnetic Probe Making Accurate Settling-Time Measurements Using a Vector Network Analyzer Applying Metabolomics Methods to the Study of Bacterial Leaf Blight in Rice Plants Measuring Stream Dynamics with Fiber Optics survey

Agilent Measurement Journal - Issue 3 - 2007

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