Discharge Measurements and Their Interpretative Value

Discharge is the measure of the amount (volume) of water emerging from a spring, or pumped from a well, per unit time. A wide array of discharge units exist in the literature, but cubic meters per hour (m3/h) is recommended. Discharge may, in certain cases, be measured in the field with the aid of a container of known volume and a stopwatch. The discharge is calculated from the volume (number of times a vessel is filled) and the time involved. Occasionally spring water flows from several directions, the output is too large, or the well discharges into a closed system of pipes. In such cases the discharge information has to be obtained from the well operator or local water authority.

Discharge of a spring or a well is a most informative parameter because it provides insight into the quantitative aspects of groundwater hydrology. Figure 4.24 includes monthly measurements of discharge, temperature, and chlorine content for a spring. Constant discharge is observed, indicating that water is delayed in the aerated zone, flowing through a porous medium (in conduit-dominated recharge, seasonal recharge fluctuations are reflected in variations in spring discharge). In addition, constant discharge indicates that the water storage capacity of the system seems to be large compared to the annual recharge or discharge. The

Fig. 4.24 Monthly measurements of discharge, temperature, and chlorine concentration in a (hypothetical) spring. The three parameters are constant over the year, indicating only one type of water is involved, recharged through a porous medium (nonkarstic), and the system's storage capacity is large compared to the annual recharge and discharge.

Fig. 4.24 Monthly measurements of discharge, temperature, and chlorine concentration in a (hypothetical) spring. The three parameters are constant over the year, indicating only one type of water is involved, recharged through a porous medium (nonkarstic), and the system's storage capacity is large compared to the annual recharge and discharge.

accompanying temperature and chlorine values are also steady, indicating one type of water is involved in the system. Figure 4.24 shows two useful modes of presentation of recharge data: as a time series and as a function of other measured parameters. The conclusion that one type of water is involved can be deduced from the horizontal line in the time series diagram or from the narrow cluster of values in the parametric diagrams.

Data plotted in Fig. 4.25 are from a different case, but resemble the parameters reported in Fig. 4.24. The obtained pattern is different: the discharge varied in an annual cycle, but the temperature and chlorine content remained constant; thus the interpretation is again of one type of water being involved, but recharge is via conduits of a karstic nature. Temperature is close to the average annual surface temperature, and fluctuations in the recharge water temperature are damped by intermixing in the aquifer. Thus the storage capacity of the system is large as compared to the peak recharge.

A third example, given in Fig. 4.26, reveals a case in which recharge varied seasonally in a spring, and temperature and chlorine content varied

Fig. 4.25 Monthly measurements of discharge, temperature, and chlorine in a spring. Temperature and chlorine are constant, revealing one type of water is involved. The significant discharge variations indicate recharge is fast and of a karstic nature.

as well, but in an opposite pattern (right diagrams in Fig. 4.26). The data plot along straight lines in the left diagrams of Fig. 4.26, revealing a negative correlation between recharge and temperature or chlorine content. Such correlation lines indicate intermixing of two water types, a topic fully addressed in section 6.6. A warmer and more saline type of water intermixes (in varying percentages) with a colder and less saline water. The highest possible temperature of the warmer end member might be deduced by extrapolation of the best-fit line to zero discharge on the temperaturedischarge graph. A value of 20.3 °C is obtained. In a similar way, the highest possible value of chlorine concentration for the warm end member may be deduced by extrapolation to zero discharge in the chlorine-discharge diagram in Fig. 4.26. A value of 420 mg Cl/l is obtained. As zero discharge has no meaning in our context (a dry spring has no temperature or chlorine

Fig. 4.26 Monthly measurements in a spring. Discharge, temperature, and chlorine are seen to vary considerably, indicating more than one type of water is involved (e.g., base flow and seasonal additions) and recharge is karstic. The data plot along straight lines in the temperature-discharge and chlorine-discharge graphs, indicating two end members are intermixing in the spring system. The temperature and chlorine values of the warm and more saline end member may be deduced by extrapolation of the best-fit lines to zero discharge (see text).

Fig. 4.26 Monthly measurements in a spring. Discharge, temperature, and chlorine are seen to vary considerably, indicating more than one type of water is involved (e.g., base flow and seasonal additions) and recharge is karstic. The data plot along straight lines in the temperature-discharge and chlorine-discharge graphs, indicating two end members are intermixing in the spring system. The temperature and chlorine values of the warm and more saline end member may be deduced by extrapolation of the best-fit lines to zero discharge (see text).

concentration), it is clear that the true intermixing water end members have values that lie between the extrapolated values (20.3 °C, 420 mg Cl/l) and the highest observed values (19 °C, 400 mg Cl/l). In this example the two sets of values are very close. The topic of water mixtures is discussed in sections 6.6 and 6.7. Discharge measurements often provide negative correlation lines, essential in finding end member properties.

A case study from the Yverdon spring, western Switzerland (altitude 438 masl) is summarized in Fig. 4.27. Maxima in the discharge curve are seen in general to be accompanied by minima in the temperature and electrical conductivity (reflecting salinity) curves, indicating a warm and more saline water intermixes with a colder and fresher water. The discharge and temperature data of the Yverdon spring have been replotted in Fig. 4.28. The best-fit line, extrapolated to zero discharge, intersects the temperature axis at 30 °C. This is a maximum possible temperature of the warm end member in the Yverdon complex. The true end member temperature is between this extrapolated value (30 °C) and the highest observed temperature (24.3 °C).

Fig. 4.27 Discharge, temperature, and conductivity (reflecting salinity) in a spring at Yverdon, western Switzerland (Vuatax, 1981). Temperature and conductivity varied in a negative correlation to discharge (Fig. 4.28), indicating mixture of two end members (see text).

Discharge, l/min

Fig. 4.28 Temperature-discharge data (from Fig. 4.27) of the Yverdon spring, western Switzerland. The negative correlation enables one to deduce an upper limit for the temperature of the warm end member (30 °C) by extrapolating to zero discharge (see text).

Discharge, l/min

Fig. 4.28 Temperature-discharge data (from Fig. 4.27) of the Yverdon spring, western Switzerland. The negative correlation enables one to deduce an upper limit for the temperature of the warm end member (30 °C) by extrapolating to zero discharge (see text).

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