• Rob Van Kirk

Diversion and Streamflow in the Henry's Fork: Why Doesn't Lower Diversion Lead to Higher Streamflow?

Updated: Jul 20

  • Due to increases in irrigation efficiency, annual diversion in the Henry’s Fork watershed has decreased by 215,000 ac-ft (20%) since 2000.

  • But, this has been offset by an equal decrease in stream reach gain, which consists primarily of groundwater return flow.

  • Mean water budget in the Henry's Fork has remained constant since 1978, roughly:

  • Inflow = 2,450,000 ac-ft

  • Outflow = 1,620,000 ac-ft

  • Net diversion = net watershed withdrawal (inflow minus outflow) = 830,000 ac-ft

  • Net withdrawal = 330,000 ac-ft consumptive use + 500,000 ac-ft Eastern Snake Plain Aquifer recharge

  • Even with equal streamflow, loss of groundwater inputs increases water temperature and decreases supply for irrigators.

  • Managed aquifer recharge can help increase groundwater inputs to the river, benefitting water users and fisheries.

  • Irrigation year 2020 aligned closely with expectations:

  • Total diversion was 98% of the 2001-2019 average

  • Inflow and outflow were both 92% of the 1978-2019 average.

  • Net diversion was 97% of the 1978-2019 average, while net watershed withdrawal was 96% of the 1978-2019 average.

  • Lower-watershed stream reach gain was 104% of average.

Irrigation efficiency

Irrigation methods changed substantially in the upper Snake River basin between the 1980s and 2000s from flooding and other direct surface-irrigation methods to sprinklers. Although unlined canals still remain the dominant method of conveyance of water from the river to fields, some irrigation companies in the Henry’s Fork watershed have converted their old canal conveyance systems to pipelines. Watershed-wide, these conversions have increased irrigation efficiency, defined here in broad terms as the amount of water used by crops as a fraction of water diverted from the river. Irrigation efficiency is the subject of much recent scientific literature and discussion among scientists and water managers. Without delving into the details, the consensus among most who study this subject carefully is that increased irrigation efficiency generally increases consumptive use of water by crops for a variety of economic, legal, and physical reasons. This is called the paradox of irrigation efficiency. In short, increased efficiency does not save water but instead usually increases water use.

Diversion, return flow, and water balance in the Henry’s Fork

One of the most important features of this paradox in the Henry’s Fork watershed is that more efficient irrigation requires less diversion from the river, which intuitively should result in higher streamflow. Instead, less diversion from the river has not increased the amount of water in the river because pipes and sprinklers result in less seepage into aquifers, which in turn reduces the amount of water that returns to the river through groundwater. Interactions between groundwater and surface water are extremely important to water management in the upper Snake River basin, particularly in the lower Henry’s Fork watershed. Because of this, I conducted a very careful and detailed analysis of trends in diversion and groundwater return flow in the watershed, comparing those trends against long-term trends in water supply (natural flow) and in regulated streamflow at the bottom of the watershed. The complete analysis is contained in the document linked here.

This blog presents the primary results of that analysis, illustrated by key graphics that plot irrigation-year 2020 observations on existing data from the 1978-2019 record. The 2020 data are preliminary and based on real-time observations. Final irrigation data are not published and approved by Water District 1 until March. Because I used preliminary 2019 data in the analysis, the numbers reported here differ slightly from those in the linked document.

I should also mention that London Bernier, an HFF intern from St. Lawrence University in 2020, conducted some more detailed analysis of reach gains in the Henry’s Fork watershed. She presented her findings at HFF’s summer seminar series in August. You can see her presentation here.

Diversion decreased abruptly by 220,000 ac-ft in 2001

Although diversion declined slowly but continuously throughout the 1980s and 1990s, the largest decrease in diversion occurred between 2000 and 2001. Mean annual diversion was 1,090,852 ac-ft from 1978-2000 but has averaged only 868,136 ac-ft since then, a decrease of 222,780 ac-ft (20%). Following that abrupt decrease, diversion has showed no statistically significant trend. Diversion in 2020 was 850,350 ac-ft, 98% of average. This figure does not include diversion into the Crosscut Canal, since that water is delivered into the Teton River, where it is diverted again. I include Crosscut diversion in my daily reports during irrigation season, because that diversion directly affects need for Island Park Reservoir draft and streamflow in the Henry’s Fork downstream, but in retrospective watershed-scale analysis I omit the Crosscut diversion to avoid double-counting. For reference, mean annual delivery of water from the Crosscut Canal to the Teton River was 45,527 ac-ft from 1978-2000 but only 37,524 ac-ft between 2001 and 2019, reflecting the watershed-wide drop in total diversion.

Importance: Irrigation practices changed dramatically in 2001 but not since then. Thus, current irrigation data are most appropriately compared against a period of record that begins in 2

The graph below shows that diversion during irrigation year 2020 closely followed the 2001-2019 average. The primary differences are lower-than-average diversion during spring rains and higher-than-average diversion during late summer and fall, which were very dry.

Reach gain in the lower Henry’s Fork decreased abruptly by 212,170 ac-ft in 2001

Water managers in the upper Snake River basin define “reach gain” as net change in streamflow in a particular reach, after accounting for diversions and reservoirs in the intervening reach. In a reach without reservoirs, for example the Henry’s Fork between Ashton Dam and St. Anthony, reach gain is calculated by the formula

reach gain = inflow - outflow + diversion

If reach gain is positive, water flowed into the reach. If reach gain is negative, the reach lost water. In the lower Henry’s Fork, reach gains and losses almost exclusively occur via interaction with the shallow aquifer. Gains occur when groundwater flows into the river, and losses occur when water flows from the stream channel into the aquifer. For watershed-scale purposes, I define lower watershed reach gain as watershed outflow (streamflow in the Henry’s Fork at Rexburg) minus lower-watershed inflow (Henry’s Fork at Ashton plus Fall River upstream of all diversions plus Teton River downstream of Crosscut Canal) plus total diversion from those river reaches. Thus, lower-watershed reach gain is the net gain/loss of water in the lower reaches of the Henry’s Fork, Fall River, and Teton River as they flow through the irrigated regions of the watershed.

Reach gain dropped by roughly the same amount as diversion did, and at the same time. Annual reach gain in the lower Henry’s Fork watershed averaged 247,677 ac-ft from 1978-2000 but only 35,507 ac-ft from 2001-2019. Reach gain in 2020 was 36,935 ac-ft, 104% of the 2001-2019 average. Although 4% above average seems like a substantial amount of water, that 4% is only 1,428 ac-ft over the whole irrigation year, equivalent to 2 cfs.

Importance: Diversion and reach gain have decreased by the same amount.

The graph below shows that reach gain during irrigation year 2020 closely followed the 2001-2019 average and illustrates the overall pattern of net loss from the river during the winter and net gain during irrigation season.

Reach gain is highly correlated with diversion

Given that reach gain and diversion both dropped by the same amount at the same time, we would expect that the two are highly correlated on a year-to-year basis. In fact that it is the case. Many decades of careful field measurements and modeling, including those made by two of my graduate students 10 years ago, clearly document a physical mechanism (irrigation seepage and groundwater flow) linking reach gains and diversion. Thus, in this case, correlation really is causation.

Importance: In the lower Henry’s Fork watershed, reach gain is essentially equivalent to irrigation return flow, that is, water that was diverted for irrigation but returned to the river before it could exit the watershed either as groundwater outflow or through evapotranspiration.

No long-term trends are apparent in net diversion.

Net diversion is equal to gross diversion minus surface return flow and measures the net amount of water that has been diverted from the surface-water system. In the Henry’s Fork watershed, net diversion is the difference between total diversion and reach gain. Given high correlation and nearly identical temporal trends in the two, we should not expect to see any trends in net diversion over time. Indeed this is true. Net diversion averaged 839,375 ac-ft from 1978-2019, with only relatively small variability around that long-term average. Net diversion in 2020 was 813,415 ac-ft, 97% of average, in line with total diversion in 2020 relative to average.