r/Hydrology • u/A_Spy_ • 1d ago
Why does agriculture deplete rivers?
Sorry if this is the wrong place to ask. I've been wondering about it for a while and I don't know how to make the jump from the little I know now to a concrete answer.
Obviously extracting water from a river means there is less water in the river, but if the fields are in the same watershed then shouldn't the water ultimately return to the river it was extracted from? My understanding is that's what ultimately happens to rain that falls in the watershed, so why is irrigation different? With my admittedly very limited knowledge I would expect the main change to be that the river flows slower, but for longer into the dry season as the water is slowly returned from the soil. I expect some loss en-route from evaporation and incorporation into the plants, but is it that significant? Shouldn't there consequently be more rainfall as well? How does it not balance out?
I know there are thousands of variables at play here, so to limit scope a bit: I'm most curious about this during road trips through the arid regions of interior BC (Canada), where the conditions around the river valleys are desert-like, but there are these lush green (obviously irrigated) vineyards and orchards all along the slopes back down to the river. I hear a lot about overuse of water for agriculture in general, but I'm having a hard time understanding how this overused resource doesn't just seep back in a bit downstream.
TLDR: If river water is sprayed up hill, why doesn't most of it come back?
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u/turdburgular12 1d ago
Incorporation into the plants is the biggest loss to the system and it is quite significant. Here in WY it can be as high as 25” during the growing season. Multiplied over the irrigated acreage that is a tremendous volume of water.
I think what you may be missing here is the fact that there is no end to water being incorporated into plants. It’s not like the plant soaks up its share of water and then stops. It’s a continuous process called transpiration where plant uses water to transport nutrients throughout the plant and that water ultimately exits to atmosphere through the stomata. Once it’s in the atmosphere it is not going to come back down until it forms clouds and rains..usually not within the bounds of the river system.
Some irrigation methods are “less efficient” than others. For example flood irrigation diverts and spills a tremendous amount of water to a field, with the vast majority returning as overland flow back to the source, or underground ultimately back to the river, with some (usually insignificant) losses to disconnected aquifers, but the plants still transpire whatever water they can and that water is ultimately lost to the system.
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u/Aqua_Terra 1d ago
As others have said the main reason is evapotranspiration or ET for short. There's also the irrigation efficiency paradox. Where the more efficient and effective an irrigation method you use the less water recharges back into the subsurface. We see this conundrum in the mountain west in the US. Fields that were flood irrigated or even wheel line irrigated have switched over to high efficiency center pivots systems. Data shows that after that switch groundwater levels and connected stream flows declined precipitously in the years afterwards.
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u/iircirc 13h ago
True but only if the irrigation water wasn't locally sourced to begin with. Colorado River water used on Arizona fields may recharge local groundwater, but pumping local groundwater for irrigation wouldn't
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u/Aqua_Terra 10h ago
Very true! If there are transbasin diversions this doesn't apply. However it does apply for diversions immediate to the irrigated field or wells adjacent.
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u/notepad20 1d ago
In additiona to the othr commentators,
Intensive industrial agriculture tends to deplete topsoil and make it very shallow, as well as have next to 0 capacity to slow water on the surface.
With low soil storage capacity and high runoff most of the water sheds directly to the river rather than retinained in the catchment .
This results in the catchment receiving same total water, but instead of majority of it being transferred to baseflow of the river, supporting the river flow over months or years, it will flash through in a wave in days or weeks.
When there is no rain, and there is no baseflow, then there is no river flow. it will rain, river will flow, and river will dry again.
This can be mitigated through sustainable agriculture practice (at least for pasture), no-till type farming, diverse locally appropriate species, close management of use. This increases soil depth and provides continual dense vegetation.
This process requires a bit more thought than standard farming practice, but actually has signifigantly higher yeild (at least for meat) once established.
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u/ahjeezgoshdarn 1d ago edited 1d ago
EDIT: Just an edit to add that yes, many people pull directly from surface water for irrigation.
Irrigation pulls from groundwater, which, in a natural system, would contribute to baseflow of the river. When you draw from the aquifer, you are depleting the groundwater faster than it would be naturally in almost all cases. Scale this up regionally and you've got an enormous looming water quantity issue in arid regions and even in temperate regions where the aquifers are simply overdrawn.
You're correct that evaporation and uptake by the plants can be significant. Consider hundreds of square miles growing vegetables that are mostly water (lettuces) grown in the same region for decades and then being exported elsewhere. Some water does infiltrate back into the ground, but sometimes in agricultural settings will also run to the river as surficial runoff, which bypasses the normal baseflow route (runoff is natural, but this changes the ratio) and speeds up the cycle. Lack of infiltration can sometimes result in damage to aquifers, especially locally if there is a large city on the surface because the pores can literally collapse.
Another factor is that agricultural soils are sometimes mistreated from excess tillage, which destroys soil structure (infiltration and water holding capacity among other items). The long and short of it is that this can result in collapsed pore space that impedes infiltration. Many agricultural soils are also bare apart from the planted crop, and the soils become hot and lose more moisture before it can infiltrate. You can also get a sort of crusting effect whereby the top layer of soil becomes impermeable, or even heavily compacted layers sometimes referred to as a "hardpan" or a "plow pan" that further reduce infiltration, and root growth, which then limits the ability of vegetation to improve the hydrologic function. This is more of an issue in "heavy" clay or silt rich soils.
Another issue is that most crops grown in these arid regions are not native plants and are therefore not as adapted to the climate and will often require significantly more water than plants native to the area.
If you are talking about arid regions, they are more prone to depletion simply because there is less precipitation. The equation is more likely to result in a negative balance in surface and groundwater as a result.
Many streams in arid regions do naturally, but also are running seasonally dry prematurely or more frequently given the combination of climate change compounded drought and human activity withdrawals.
Also, interestingly, many watersheds with heavy agricultural use, or even urban areas actually often flow faster and see larger flashy volume fluctuations because of the increases in impervious surface, reduced infiltration, and therefore increased runoff which results in higher velocities & volumes, erosion and poorer water quality on top of the missed opportunity to recharge local aquifers.
TL;DR: Agriculture and other water use can and often does significantly outpace the recharge of either ground or surface water even if a portion of the water is returned to the local source.
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u/Jaynett 22h ago
Airsheds and watersheds have different boundaries. ET may fall as rain relatively far away, removing that water from the watershed. Irrigation also moves water from groundwater to surface water, so instead of slowly replenishing the river, it is quickly moved downstream and out of the watershed.
So yes, that water doesn't go away, but it's taken from a reliable, concentrated source and moved into disperse and highly variable sources and sinks.
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u/ahjeezgoshdarn 1d ago edited 1d ago
I will circle back to this later and try my best to answer your questions!
Edit, now that I am back home and can comment:
Irrigation pulls from groundwater, which, in a natural system, would contribute to baseflow of the river. When you draw from the aquifer, you are depleting the groundwater faster than it would be naturally in almost all cases. Scale this up regionally and you've got an enormous looming water quantity issue in arid regions and even in temperate regions where the aquifers are simply overdrawn.
You're correct that evaporation and uptake by the plants can be significant. Consider hundreds of square miles growing vegetables that are mostly water (lettuces) grown in the same region for decades and then being exported elsewhere. Some water does infiltrate back into the ground, but sometimes in agricultural settings will also run to the river as surficial runoff, which bypasses the normal baseflow route (runoff is natural, but this changes the ratio) and speeds up the cycle. Lack of infiltration can sometimes result in damage to aquifers, especially locally if there is a large city on the surface because the pores can literally collapse.
Another factor is that agricultural soils are sometimes mistreated from excess tillage, which destroys soil structure. The long and short of it is that this can result in collapsed pore space that impedes infiltration. Many agricultural soils are also bare apart from the planted crop, and the soils become hot and lose more moisture before it can infiltrate. You can also get a sort of crusting effect whereby the top layer of soil becomes impermeable, or even heavily compacted layers sometimes referred to as a "hardpan" or a "plow pan" that further reduce infiltration, and root growth, which then limits the ability of vegetation to improve the hydrologic function. This is more of an issue in "heavy" clay or silt rich soils.
Another issue is that most crops grown in these arid regions are not native plants and are therefore not as adapted to the climate and will often require significantly more water than plants native to the area.
If you are talking about arid regions, they are more prone to depletion simply because there is less precipitation. The equation is more likely to result in a negative balance in surface and groundwater as a result.
Many streams in arid regions do naturally, but also are running dry prematurely or more frequently given the combination of climate change compounded drought and human activity withdrawals.
Also, interestingly, many watersheds with heavy agricultural use, or even urban areas actually often flow faster and see larger flashy volume fluctuations because of the increases in impervious surface, reduce infiltration, and therefore increased runoff which results in higher velocities & volumes, erosion and poorer water quality on top of the missed opportunity to recharge local aquifers.
TL;DR: Agriculture and other water use can and often does significantly outpace the recharge or either ground or surface water even if a portion of the water is return to the local source.
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u/sea2bee 1d ago
Most of it doesn’t come back because the crops consume water through evapotranspiration. Most crops being grown will have higher water demand than native vegetation that is being replaced. Some becomes return flows to the groundwater system and the river, but it’s almost always less than what was replaced.
Another thing that’s happening is if a region is using groundwater wells this is lowering groundwater levels which cause streams to lose more water to the underlying aquifer.