# Delineation of catchment zones of geothermal systems in large‐scale rifted settings - **Authors:** D. E. Dempsey; S. F. Simmons; R. A. Archer; J. V. Rowland - **Journal:** Journal of Geophysical Research: Solid Earth, vol. 117(B10), article 2012JB009515 (2012) - **Publisher:** American Geophysical Union (AGU) - **DOI (version of record):** [https://doi.org/10.1029/2012jb009515](https://doi.org/10.1029/2012jb009515) - **This page:** https://ddempsey.github.io/papers/2012-dempsey-delineation-catchment-zones-geothermal-systems/ - **Open PDF:** [https://ddempsey.github.io/papers/2012-dempsey-delineation-catchment-zones-geothermal-systems/dempsey2012-delineation-catchment-zones.pdf](https://ddempsey.github.io/papers/2012-dempsey-delineation-catchment-zones-geothermal-systems/dempsey2012-delineation-catchment-zones.pdf) (Published version) - **Full text (Markdown):** [https://ddempsey.github.io/papers/2012-dempsey-delineation-catchment-zones-geothermal-systems/fulltext.md](https://ddempsey.github.io/papers/2012-dempsey-delineation-catchment-zones-geothermal-systems/fulltext.md) - **Notice:** Published version, posted under AGU's author self-archiving policy. © 2012 American Geophysical Union. All Rights Reserved. Version of record: https://doi.org/10.1029/2012jb009515 - **Explainer:** all explainers on this site are AI-generated. - **BibTeX:** https://ddempsey.github.io/papers/2012-dempsey-delineation-catchment-zones-geothermal-systems/citation.bib - **Topics:** Volcanoes & eruption forecasting; Geothermal systems - **Keywords:** Geothermal gradient; Geology; Convection; Crust; Volcano; Geophysics; Petrology ## In brief We modelled circulation in a rift like New Zealand's Taupō Volcanic Zone and traced water from ten modelled geothermal fields to where it entered the ground. Their catchments cover 70 to 500 km² and increase with field heat output. Most water spends 5,000 to 50,000 years underground. - We traced 1.7 million flow paths backwards through a 3-D model of rift-scale circulation, from ten modelled geothermal fields to where their water entered the ground. - Most water spent 5,000 to 50,000 years underground, and water younger than 5,000 years entered the ground within 1 to 2 km of a field. - Catchments of 70 to 500 km² overlapped little, were asymmetric about their fields, and increased in area in proportion to field heat output. - A Voronoi construction based only on field positions and heat outputs approximately reproduced the main features of the modelled catchments. - Applied to the TVZ, the method leaves an empty region near the Horohoro prospect, large enough for the catchment of a system of up to 400 MW. ## The problem In New Zealand's Taupō Volcanic Zone (TVZ), hot water rises in more than 20 plumes, and several of these host reservoirs used for heat and electricity. Most of the water starts as rain. It percolates down through the crust, is heated at depths of 6 to 8 km, and returns to the surface at a field (McNabb, 1975; Bibby et al., 1995). Because the chemistry of geothermal water reflects where it entered the ground, knowing a field's catchment can help to interpret that chemistry. As with the catchment of a water well (Frind et al., 2006), the catchment of a geothermal field is the area from which water can travel to it. However, a catchment can be delineated only where the flow underground is known. Our goal in this study was to estimate where geothermal catchments lie, how large they are, and how long their water spends underground. ## What we did We used FEHM (Zyvoloski, 2007), a computer code for the flow of heat and water through rock, to model a block of crust 50 km across the rift, 80 km along it, and 8 km deep, heated from below. We added random variations in permeability to represent the uneven distribution of fractures. After 500,000 simulated years, ten separate fields had formed, with heat outputs of 44 to 833 MW. However, the model represents a rift with average TVZ properties. Its fields do not correspond to individual TVZ fields. We then released particles, computational markers that move with the modelled water, inside each field and tracked them backwards in time. A random step at each move represented dispersion, the spreading of water between neighbouring flow paths. We built 1.7 million flow paths and defined the catchment of each field as the area of ground where its paths begin. ## What we found For one field of 105 MW, most water spent 5,000 to 50,000 years underground, with a mean of 22,300 years. Across the ten fields, mean residence times ranged from 12,000 to 33,000 years. Water younger than 5,000 years came almost entirely from within 1 to 2 km of a field. Water age also varied within a single field. We therefore caution against treating the water of a field as a single, well-mixed fluid. Catchment areas ranged from 70 to 500 km² and overlapped little. Most were asymmetric about their field. This is because neighbouring catchments abut, and recharge is concentrated where permeability is high. Dispersion also enlarged the catchments. Without it, the catchment of the 105 MW field fell from 176 to 35 km². Catchment area increased in proportion to field heat output. A simplified analytical model that treats flow in one dimension (Weir, 2009) predicts that a field gathers 0.5 to 1 MW of heat per km² of catchment. A straight line fitted to heat output against catchment area for our ten fields and those of four other model runs has a slope of 1.38 MW per km². The two agree in order of magnitude. Individual fields scatter about this line, and the 105 MW field has about 0.6 MW per km². For a 200 MW field, the 1-D model gives a catchment of 200 to 400 km², a square about 14 to 20 km on a side.
We also built Voronoi diagrams, growing a region outward from each modelled field, at a rate set by its heat output, until the regions met. The result reproduced the main features of the particle-tracking catchments. It is a first-order approximation, because it ignores permeability and dispersion. However, it needs only field positions and heat outputs. For TVZ fields, we used heat outputs from Bibby et al. (1995). The method suggests, for example, that Orakeikorako takes most of its water from the west. Rain in the TVZ contains less deuterium, a heavy form of hydrogen, towards the south (Stewart and Taylor, 1981). Reservoir waters from nine fields show the same trend (e.g., Giggenbach and Stewart, 1982; Stewart, 1986). This suggests that recharge is local. The method also left an empty region centred on the Horohoro geothermal prospect, whose heat output of about 4 MW (Allis, 1987) is too small to account for the gap. The crust there may be too impermeable for water to circulate through it, or the Horohoro resource may have been underestimated. The gap is large enough for the catchment of a system of up to 400 MW. In Iceland, on the Reykjanes Peninsula, several Voronoi catchments extend into the ocean. This is consistent with seawater in those geothermal fluids. Chloride reaches about 19,600 parts per million at Reykjanes, against 50 to 100 parts per million further inland (Arnórsson and Gunnlaugsson, 1985; Arnórsson, 1995). ## Limitations Our modelled fields have lower average heat outputs than TVZ fields, and a maximum temperature near the top of the model of 250°C, against about 300°C in the TVZ. The model omits magmatic fluid, which may supply up to 6% of the water in western fields and 14% in eastern fields (Giggenbach, 1995). It also ignores topography, which affects groundwater flow and rainfall distribution. Our mean residence time of 22,300 years for one field is higher than the 12,000-year upper limit estimated for deep water at Wairakei (Stewart, 1978). The TVZ heat outputs used for the Voronoi catchments have standard errors of about 50% (Bibby et al., 1995). The resulting TVZ catchments are therefore approximate. We expect that including topography will further constrain where geothermal catchments lie. ## Glossary The **Taupō Volcanic Zone (TVZ)** is the rifted region of active volcanism and geothermal activity in New Zealand's central North Island. A **geothermal field** is an area where hot water from depth reaches the surface, defined in our model by heat flow above 2 W/m². **Recharge** is the water that enters the ground and eventually supplies a geothermal field. A **catchment** is the area of ground surface from which a field's recharge is drawn. **Residence time** is the time water spends underground between entering the ground and returning to a field. **Permeability** is a measure of how easily water flows through rock. **Dispersion** is the spreading of water between neighbouring flow paths as it moves through rock. **Heat output** is the total heat leaving a field's surface, in megawatts (MW), where 1 MW is a million watts. A **Voronoi diagram** divides a map into regions, each containing the area closest to one of a set of points. In our weighted version, each region grows outward from its field at a rate set by the field's heat output. Larger fields therefore have larger regions. **Deuterium** is a heavy form of hydrogen whose abundance in rain varies with latitude. ## Abstract Hydrothermal convection in the Taupo Volcanic Zone (TVZ), New Zealand, is driven by heat extracted at the brittle‐ductile transition, which is in turn supplied by magmatic intrusion of the lower‐crust. We present a numerical model that approximates this circulation in a statistical sense, being constrained by TVZ dimensions and mean thermal properties, but incorporating a permeability distribution of arbitrary heterogeneity. A particle tracking methodology that accounts for advective and dispersive transport is introduced, and ensembles of several tens of thousands of flow paths are constructed for each of the modeled geothermal systems. These flow paths reveal the nature of mass recharge in multicell convective systems, and suggest the ages of waters in TVZ geothermal systems vary between 5 and 50 kyr. Flow path ensembles are used to delineate catchments for each of the modeled systems and a method for calculating their areas is introduced. Catchment area is shown to be proportional to system heat output and area, which is consistent with a 1‐D analytical model of heat and mass transfer. As an approximation to catchments delineated by particle tracking, we outline a method of Voronoi tessellation based on the positions and heat outputs of geothermal systems. This method is used to delineate catchments for geothermal systems in the TVZ and Iceland. ## Cite Dempsey, D. E., Simmons, S. F., Archer, R. A., Rowland, J. V. (2012). Delineation of catchment zones of geothermal systems in large‐scale rifted settings. Journal of Geophysical Research: Solid Earth, 117(B10), 2012JB009515. https://doi.org/10.1029/2012jb009515 ```bibtex @article{dempsey2012delineation, title = {Delineation of catchment zones of geothermal systems in large‐scale rifted settings}, author = {Dempsey, D. E. and Simmons, S. F. and Archer, R. A. and Rowland, J. V.}, journal = {Journal of Geophysical Research: Solid Earth}, year = {2012}, volume = {117}, number = {B10}, pages = {2012JB009515}, publisher = {American Geophysical Union (AGU)}, doi = {10.1029/2012jb009515}, url = {https://ddempsey.github.io/papers/2012-dempsey-delineation-catchment-zones-geothermal-systems/} } ```