Dave, Don, Pete and Budyong had a two hour meeting at MDC offices on Wednesday, August 23rd. Peter Davidson, Groundwater Scientist organised the meeting to give us a presentation bringing us up to date on the latest research about the aquifer and its decline and sharing some of their thinking about how they might better manage the water allocations being extracted from the aquifer. Other staff attending were Pere Hawes, Sarah Pearson and Clementine (Clem) Rankin.
Peter gave a comprehensive Powerpoint presentation. Initially he stated that their aim in regard to aquifer water is -
“Certainty for water users, community & environment (not there yet)”
As you can see by from Peter’s words in brackets there is still work to do to provide the certainty they are aiming for and they are currently “refining some limits & balancing water budgets”.
For context it is important to note the “Hierarchy of Needs” as expressed in Te Mana o te Wai is -
They recognise that -
Note these slightly lower flows are from data over the last 65 years. The graph shows only a small downwards trend over that time. The major cause of the decline in the aquifer is not so much the flows but the changes to the river bed in the recharge zone, as I reported last year.
What has become very clear is that the aquifer is a very dynamic system. It refills/drains quickly in response to Wairau River floods/low flows. At the end of August 2022 it was recorded at the Condors Bend monitoring bore as being at it’s highest level ever recorded and then by February this year it is was at its lowest level ever recorded for that time of year. They therefore believe it is valid to manage the entire aquifer from the Waihopai River to the sea as a single system using a real time management approach.
What is very important to understand is that the aquifer storage volume that is available for allocation is only about 10% of the total. In other words a maximum of 10% of the total can be allocated before having an unacceptable impact on environmental needs (springs/seawater
interface/maintaining gradient). For water users the reservoir is effectively empty when groundwater falls to the minimum level required to maintain acceptable spring flows.
This brings us to the major challenge the MDC team are trying to address - how best to apply limits to water extraction from the aquifer without compromising environmental needs. They have to re-interpret minimum levels required to provide for the springs (Awarua-Spring Creek) in the new context of Te Mana o te Wai.
Some limits were set when the draft Marlborough Environment Plan (pMEP) was first released in 2016 along with limits on all other rivers and aquifers in Marlborough. It is relatively simple to manage limits in rivers by measuring river flows and applying appropriate cut-offs to retain adequate flows in the rivers to meet environmental needs. As it is not possible to measure flows into the aquifer this method is not available. It can only be done by monitoring the fluctuating aquifer levels. With the Wairau aquifer being so dynamic, these fluctuations can be large in short time periods.
The Wairau aquifer did have an annual maximum extraction limit set of 73,000,000 M3. The latest data on extraction indicates up to about half of that amount (about 35,000,000 M3) is currently being extracted annually depending on the season. Additional limits aiming to protect the springs were also set on three sub-zones adjacent to the springs where cut-offs apply once the aquifer drops to pre-set levels. As I’ve reported before, with the knowledge gained from the GBR research, it is now accepted that neither of these options are seen as being fair and equitable for users or adequate for optimum springs protection. In other words the hard cut-offs in the pMEP are now seen as not viable. The water users in these sub-zones all objected but after discussions have agreed to accept the current limits as defined in the pMEP pending the completion of the process described in this paper. An annual allocation is also seen as too crude for managing seasonal variations.
Even with the knowledge that the aquifer holds less water than originally thought, it still holds a lot of water. Water enters it from the river in the recharge zone and leaves it through extraction or the springs. Council staff have to convince the 600 water users that the majority of the water in the reservoir is not available for extraction. The upside is that the effect of current abstraction is seen as acceptable under most seasonal conditions with the acknowledgement that understanding of how overall abstraction changes groundwater level is still imprecise. Most consents are now metered but there are still data quality issues. Once actual use is accurately known then the cause and effect relationship (abstraction vs groundwater level) can be defined. This is an issue across NZ and staff recognise we need to do better. Confirming the Wairau aquifer quantity limit is an essential foundation for the pMEP.
Our future climate & how it may affect catchment land use, naturally results in uncertainty. Climate variability will affect rainfall and runoff and the level of natural gravel supply for replenishing the depleted gravel bed of the river is also unknown. This means there is uncertainty if the long-term declining trend will continue, reverse or stabilise as this is directly related to natural sediment supply. The aquifer computer model has now been expanded to include the whole plain. It is a key tool for refining limits & exploring alternate management approaches. There is more confidence that the latest computer model is hydrologically correct now we have the Gravel Bed Rivers research information.
The model will be used to review the sustainability of the pMEP annual volumetric limit. The effects of water use up to the limit can be simulated by extrapolating from current use. Results will not be available until 2024. Wairau Aquifer hydrological processes are well understood but fluxes/actual use are still approximate. The MDC priority is to balance the water budget. The need for an instantaneous cumulative abstraction limit complimenting the annual limit, to avoid seasonal stress of Wairau Aquifer, is being investigated.
In conclusion MDC are considering three different options for managing limits.
1) A fixed seasonal allocation – this would be an unrestricted, set quota providing sufficient groundwater for the average irrigation season. It would be indexed against the overall aquifer storage required to maintain spring flow/environmental servicing.
This provides certainty but would be a rationed amount in lieu of seasonal restrictions. It would be simple administratively but need to be adjusted if aquifer trends upwards or downwards.
2) Fixed seasonal allocation – this would be an unrestricted, set quota indexed against the aquifer state on 1 October each season with a reduction in lieu of restrictions. It would have to be within any revised overall volumetric limit.
This provides certainty but the season may change and it would be complicated administratively.
3) Dynamic storage approach – this would be a variable monthly quota with more water available when the aquifer is full & less when empty. The quota for an upcoming month would be specified as a percentage of the consented share of the total Wairau Aquifer volume.
This provides less certainty for irrigators but better reflects the state of the aquifer with increases/decreases made when appropriate. It also accounts for climate variability. It is capable of automation but consent holders would need a tool to help plan water use.
National Policy Statement on Freshwater (NPSFM) update.
MDC are undertaking a process of community consultation as a part of their obligation to make plan changes necessary to implement the requirements of the NPSFM. The first consultation on values and overview has been completed and submissions are being analysed with a full report being presented to the Environment and Planning committee in October. CKM did put in a submission.
The main community concerns expressed are -
• Plantation forestry effects on water yield and quality.
• Wilding conifer infestations effects on water yield and quality.
• Waterway maintenance.
• Flooding hazard.
• Appropriate water allocation and minimum flow setting for healthy waterbodies.
• Pest management.
• Dealing with and understanding the changes to legislation and the regulation that goes with them.
• Lack of availability of important information so informed decisions can be made.
There are two more community engagements planned, one for later this year and one for mid 2024. The process has to be completed by December 2024.
Council are also looking to set up region wide water users' groups. There are likely to be 2 groups, 1 specifically for the Wairau being our biggest system and then a second which will have all the other area users. These groups would be used to discuss regulatory options and test/run scenarios. They are looking to start work on this process probably early 2024. They have an expectation for regular meetings/discussions.
The three members of the team overseeing this process who attended our meeting on August 23rd indicated they hoped CKM would participate in the Wairau water users group to provide an alternative perspective to commercial water users. I indicated we would see our involvement as being a voice for Te Taiao and the needs of the environment would be our interest.
A declining trend in Wairau Aquifer levels has been observed at western Wairau Plain MDC monitor wells since 1973, which pre-dates the Marlborough wine industry. While the declining trend of around 1 metre over 50 years may not appear large at an average of 20 mm/year, the risk is if it continues over the very long-term, groundwater springs may dry up and ultimately there could be seawater intrusion of our coastal aquifers. Given the depleted storage volume of the Wairau Aquifer after a series of dry summers since 2014 and minor Wairau River activity through the past winter recharge period, current levels of consented abstraction are also compounding the issue. Identifying the drivers via the Gravel Bed Rivers Project (GBR) is a priority project for MDC, given the longer term regional scale implications for flood control, reliability of consented water consents, wetland health and security of coastal groundwater supply.
The causes of the decline are known to be incremental and long-acting. These drivers have been actively investigated over the past decade, but research is challenging because most of what goes on is hidden beneath the surface with measurements often indirect in nature.
You can learn more about this project by downloading these two files from the MDC website. The original report from June 2020 is available here (Agenda item 5) and the update from April 2021 is available here (Agenda item 4).
Dave, Don and Budyong met with Pere and Hans for a couple of hours. The meeting was very congenial, relaxed and informative. We got the impression that they also appreciated the opportunity to get together for an extended discussion. The primary focus of the meeting was to discuss issues relating to the Wairau aquifer and it’s management. We had produced a document that we sent in beforehand with a range of points highlighted from the Guidance on the National Objectives Framework of the National Policy Statement for Freshwater Management (NOF) and questions arising from those points. A copy is attached FYI.
In earlier communications with Peter Davidson, the MDC Groundwater Scientist, he referred to using “alternative approaches to managing seasonal and boundary effects”. The primary objective of our meeting was to try and get a better understanding of what alternative approaches are being considered by the policy makers. This is in relation to the question of how best to continue allocating and managing water extracted from the aquifer taking into account the ongoing declining trend caused by the identified reduction in recharge. It is important to note that research has also identified that water can pass through the aquifer in as little as three months which means it is a much more dynamic and variable storage reservoir than previously thought. Traditionally the management has been done by setting physical limits and currently the three smaller sectors of the aquifer, immediately adjacent to the springs, have level limits (metres above sea level) set in the Proposed Marlborough Environment Plan (pMEP) and the two larger, Lower Wairau and Recharge Sectors, have no limits set. Knowledge has improved considerably since 2016 when these limits were set in the pMEP. Water users from the three smaller sectors are appealing these limits and it seems they have a good case to not be treated differently from all other users of water from the aquifer. The proposed limits are likely to be removed. Pere highlighted that fact that Peter Davidson believes the aquifer should be seen as a huge underground reservoir that is big enough to be managed to meet the identified and allocated needs of water users in the area. The point was again made that the major issue with the aquifer is not with water use from it but rather with the declining recharge from the river.
So it appears that the answer to the question of what “alternative approaches to managing seasonal and boundary effects” are being considered is that setting aquifer level limits is not an effective way to manage the reservoir. The management tool is likely to simply be the total volume of water allowed to be consented from that FMU, which is calculated according to what the scientists believe the aquifer can reasonably provide on an ongoing basis. Regardless of water use the decline is expected to continue the same slow downwards trend, unless the recharge can be improved. It seems counter intuitive to manage the aquifer like this but its dynamic nature means it can also fill up very quickly with big flood events. It is very different, for instance, from the Southern Valleys aquifer where the water is at least a thousand years old and replenishes slowly. In the rainfall events this past July/August the monitoring bore at Conders Bend registered the highest aquifer level recorded since records began in 1982. The blue line on the graph below showing aquifer movements this year, illustrates how fast things can change. Effectively the aquifer was above ground level in low lying places at its peak and water pooled for several weeks before the aquifer slowly returned to lower levels. I witnessed this at the Crum’s property in Inkerman St just below the terrace in Renwick. You can also see by early November it had dropped to well below average for this time of year. One of the unavoidable consequences seems to be that the headwaters of the various springs, where water exits the aquifer at ground level, will continue to slowly recede and fluctuate as a visible manifestation of the changing aquifer levels.
This all highlights the big question. Can changes be made to river management that improve the recharge rate? There are already plans to remove some of the rock walls that have traditionally controlled the river flow within what were seen as acceptable parameters. The Gravel Bed River Research (GBR) has determined that the deep pools that form against these rock walls create the opportunity for water to flow back out of the aquifer during periods of low flow in the river. So, letting the river have more room between the existing stop banks is a first step in trying to slow the aquifer decline. Pere also informed us that the option of moving stop banks, again to give the river more room to move, is being considered. There are places like Conders Bend where MDC owns the land adjacent to the stop banks and this land is currently leased for grape growing. Using river management to increase the gravel levels in the river would also be advantageous, as those river bed gravels are important for holding and feeding water into the aquifer.
There were several other interesting bits of information that came from our discussions. Hans talked about a decision he was involved with in the 1990’s that appears to have had a big part to play in directing land use in Marlborough to grape growing rather than dairying. This was when water allocation parameters were being set and many councils around the country decided to issue 30 year consents. He was advised by an economist that the best way to discourage dairying was to only issue 15 year consents, as banks were reluctant to loan on dairy conversions with the shorter consents for water use. The massive expansion of grape growing has been an outcome of this policy. We only have to look to Canterbury to see what has happened there with excessive dairy conversions on inappropriate soils and the difficulty they now have to try and turn around a whole industry so they can halt the environmental damage to their water. Any monoculture invites problems but at least our water in Marlborough is relatively healthy compared to many other areas of NZ due to the relatively low impacts of grape growing. It was also mentioned that there are only about 4,000 hectares of identified suitable grape growing land remaining in Marlborough so we are reaching the physical limits of what can be grown here. The recently retired MDC hydrologist Val Wadsworth, claimed that the water available from the Wairau River and aquifer is more than adequate to meet the needs of the water users in the valley as long as it is well managed. It was also mentioned that some of the big corporates involved in wine making are driving moves towards more sustainable grape growing and organics because of consumer pressure and better returns. They have the resources to make this happen and it seems reasonable to assume Wairau water quality will benefit from this development.
We also discussed monocultures and diversification. Pere asked me why I was concerned about our reliance on grape growing in Marlborough and my response was simply that I believed that too much reliance on one industry in any region reduces resilience if some unexpected disruption turns up. Hans said he agreed and has been trying to encourage land users in Marlborough to think about other possibilities for our soils and climate and believes there will be other crops that can give a good financial return and have low environmental impacts. Pine nuts is one local example showing promise.
Another bit of information was that MDC have commissioned NIWA to do a detailed analysis of sea level rise in Marlborough with more definition than that available in the recently released national database. This follows NIWA having completed such an analysis for the Nelson/Tasman region.
Finally there was also some discussion about challenges with involving local Maori in the Te Mana o te Wai process. For those who are not familiar with this central concept for fresh water management here is some information from the MfE factsheet -
“Te Mana o te Wai refers to the vital importance of water. When managing freshwater, it ensures the health and well-being of the water is protected and human health needs are provided for before enabling other uses of water. It expresses the special connection all New Zealanders have with freshwater. By protecting the health and well-being of our freshwater we protect the health and well-being of our people and environments. Through engagement and discussion, regional councils, communities and tangata whenua will determine how Te Mana o te Wai is applied locally in freshwater management. Te Mana o te Wai has been part of the National Policy Statement for Freshwater Management since 2014, though there are changes to how the concept is described and how it must be applied.”
The challenge MDC face is that local iwi other than Ngai Tahu have very limited resources to participate effectively in such consultation processes.
We have written to Pere and Hans with some follow up questions arising from our meeting and are currently waiting for their response.
Budyong, Don and Dave.
Here is a copy of the letter with the FOLLOW UP QUESTIONS sent to Pere and Hans on 09/11/2022.
Hi Pere and Hans,
Firstly we wish to thank you both for the opportunity to have our wide ranging discussion last week. We all found it very informative and useful and appreciated your openness and willingness to share your knowledge with us. We also appreciated that both of you have such a good big picture view of the world.
We have been putting together a report for CKM members on what we have learned but before we can complete it we are hoping you can give us clarification in writing on some points we are still unclear about. Our questions are in bold below.
Our understanding from the meeting is that it is likely after the pMEP appeals process that the existing level limits set in the Northern, Central and Urban sectors of the Wairau FMU will be removed.
We note on page 93 of the NOF it states -
"Take limits must:
• be expressed as a rule in the regional plan, and be set for every FMU
• achieve the flows and levels for an FMU. They should not allow flows or levels of water to fall below these.
• be expressed as a volume or rate of take, or both. For example: ‘No more than xx m3/s can be taken from this river from 1 November to 31 March’. Groundwater may be expressed as maximum annual volume taken.
Take limits must also:
• provide for flow or level variability that meets the needs of the water body from which the water is taken or diverted, and the needs of connected water bodies, and their associated ecosystems (eg, a groundwater take limit would not cause a permanent lowering of the groundwater levels, or reduce connected river flows below their environmental flow)"
1) Are we correct in thinking that the only likely limit for the whole Wairau Aquifer FMU will be a "maximum annual volume taken" as defined above?
2) Are we correct in assuming MDC believes the current annual limit of 73,006,000 M3 will "not cause a permanent lowering of the groundwater levels, or reduce connected river flows below their environmental flow"?
We understand the annual extraction limit in Appendix 6 was set when the pMEP was released in 2016. Subsequent information coming from the GBR research (in particular the isotopic research) has provided evidence that the reservoir volume is smaller than initially believed.
To quote from the report to the Environment Committee on Aug 29th, 2019.
"A new method has for the first time verified the age of Wairau Aquifer groundwater at less than 1 year meaning the reservoir volume is small.
This means that for the critical well levels and associated groundwater fed spring flows, the aquifer is never more than several months away from receding to MEP environmental thresholds."
If this assessment is correct, it raises another question for us.
3) Does this mean the annual extraction limit of 73,006,000 M3 set in 2016, before the isotopic research, might now need downward adjustment?
It seems to us that the need for an accurate assessment of the total volume of the aquifer is critical if we are considering having a regime where there are no level limits set in the aquifer and we rely on the annual volume limit as our only control on extraction.
4) How confident are we that our current assessment of the total aquifer volume is accurate?
5) What is the average total volume assessment?
It would be very helpful for us to know so we have an idea of what proportion 76,000,000 m3 is of the total available. We understand the concept of imagining the aquifer like a bucket where the level goes up and down. So, is it 5% of the bucket or 40% for instance? We realise this total will vary considerably for different times of the flood/drought cycle and that water is constantly flowing through the aquifer.
Our primary concern is in regard to what impacts there might be on the aquifer and its associated river and spring flows when we have aquifer water users taking their full allocations during an extended multi year drought that climate change projections indicate is becoming more likely. The emptier the bucket becomes the greater the impact will be from extracting 76,000,000 m3 annually.
We accept that the full allocation is not normally extracted each year but presumably we do need to imagine worse case scenarios.
Do we have accurate information at this point in time to inform us how empty the bucket might get in such a multi year drought scenario?
In other words would several years of removal of 76,000,000 m3/year, with annual recharge volumes lower than extraction and springs outflows combined, result in unacceptable lowering of the aquifer?
How can we meet the requirements of the NOF without having a clear definition of how empty the bucket can get before extraction is halted?
Thanks for your help. It is appreciated. Budyong, Dave and Don. (for CKM)
Response received from Pere Hawes on 22/11/2022.
Good morning Budyong, Dave and Donald.
Apologies for the delay in response. My team has had some Environment Court reporting timeframes that had to take priority.
It was a pleasure meeting with you all recently to discuss the Wairau Aquifer and its management.
I think the best way to respond to your questions is to not actually answer them at this point in time. That is not intended to be obstructive gents, but merely reflects that the Council is yet to make any decision with respect to the current PMEP take limit (or other limit) for the Wairau Aquifer. As described to you at our meeting, policy development is a process of navigation as we respond to new information and changing community expectations. We continue to work with our hydrology and ecology teams to better understand the dynamic nature of the recharge mechanism for the Wairau Aquifer. Until that work is complete, and we have greater certainty regarding the resilience of the aquifer, it is best not to formally speculate on possible management outcomes. I say this because we have a process to work through to implement the National Objectives Framework and that work also involves future engagement with tangata whenua iwi and the community. The feedback received through these processes will be influential in setting a management outcome for the Wairau Aquifer.
Similarly, in terms of the current limits for the Spring Sector, the appeals on these limits must go through a process of Court led mediation and we will not know the outcome of that process until well into the New Year.
I have forwarded your letter to Peter Davidson so that he is aware of the questions you pose. He may be able to answer some of the questions from a hydrology perspective. Peter, Hans and I understand the risks of relying upon a take limit for the Wairau Aquifer only and we will be considering the effectiveness and efficiency of other forms of limit as part of the process once the management objective is clear.
We did candidly describe possible trajectories for this work in person so that you got an appreciation of the complexity of the work we are undertaking and the implications for the resource, resource users and mauri/natural character of the water bodies.
Thank you for your ongoing interest in this topic and please keep in touch so that we can provide updates as the work progresses. Rest assured the questions that the CKM are asking are also the questions that we have.
Manager Environmental Policy
Marlborough District Council
The information below was written after a presentation was made to the MDC Environment Committee on June 15th, 2022.
Gravel Bed Rivers (GBR) research on Wairau River and Aquifer.
A presentation was made to the MDC Environment Committee on June 15th providing them with the latest information arising from the GBR research program which started in 2019. The purpose of this latest report was "To provide an update to the Committee on research results from the national Gravel Bed Rivers project investigating the hydraulic connection between braided gravel rivers and alluvial aquifers."
In their Executive Summary they state - "The prime reasons for the ongoing decline in Wairau Aquifer well levels is less Wairau River water
available for recharge and a reduction in the capacity of the natural pathways to move water from the river into the aquifer. This is compounded by demand in some drier seasons."
This is not really an unexpected conclusion when we know the river has been modified severely from it's original natural course and is now contained within stopbanks. It is interesting to note that the research team think that water extraction from the river and aquifer is not a major contributor to the ongoing decline trend and that it is the reduced recharge that is of the greatest significance.
The research team have proposed - "Having established a conceptual model of how the river-groundwater system work, the river-groundwater system will be modelled more accurately than previously. A model will be used to test the sensitivity of the river-groundwater water balance to riverbed elevation, scouring, and floodway width. The results will be used as a basis for a cost-benefit analysis to see how changes to current river management would impact the local economy."
If effective solutions can not be found to stop the decline in the aquifer the consequences for those growing and processing grapes and others who rely on this water for their operations and livelihoods will, in time, be considerable. Add to this the prediction that we are likely to experience hotter and drier summers due to global warming and it is not hard to imagine serious impacts for Marlborough in the decades ahead.
I recommend reading the full Executive Summary (Item 5 on the agenda) if you wish to understand more clearly the dynamics of the reduced aquifer recharge process proposed by the research team.
It is of further concern to read that "The decline in Wairau Aquifer levels is consistent with widespread deepening of wells over the past
35 years at least. Deepening wells improves individuals access to groundwater but will not prevent aquifer fed springs from drying up as they rely on shallow groundwater breaking the surface for their existence."
At the same meeting MDC Groundwater Scientist Peter Davidson also presented the annual Groundwater Quantity State of Environment (Item 6 on the agenda) report. One bit of information from the report (page 12) stood out for me. Peter believes that "based on an extrapolation of the current rate of flow recession, Spring Creek will recede to State Highway 1 by about the year 2100 and by association all of the springs including in Blenheim." I see this as concerning information. One thing our communications with MDC staff have highlighted is that there is not enough evidence of the actual volumes of water being drawn out of the aquifer by water users to ascertain yet how much this water use is impacting the declining trend in the aquifer, as actual water use has only been metered for the last 5 years or so.
The lower Wairau aquifer has 3 Freshwater Management Units (FMU's). Levels are set, that if reached, will trigger restrictions for water users in those areas. For anyone interested you can view the Graphs showing the cut-off levels for the Northern (Wratts Rd), Central (Mills and Ford Rd) and Urban (Murphy's Rd) springs FMU's. You can also access the graphs showing long term data from the monitoring wells on the council website here. I have analysed some data supplied by MDC and it is interesting to note that in the dry years of 2015, 2019 and 2020 the aquifer level in the Northern (Wratts Rd) monitoring bore was only 50 - 60mm above the restriction level. This is the bore closest to the Spring Creek headwaters and therefore the best indicator of likely impacts on the springs. In communication with council staff we have learned that there are restrictions on all Wairau Aquifer Sectors except for the Lower Wairau and what they call the Recharge Sector of the main aquifer, which is a large proportion of the total aquifer. The reason the Recharge Sector has no restrictions currently is that MDC weren’t confident at the time of writing the MEP (Marlborough Environment Plan) that they had sufficient understanding of whether reducing cumulative abstraction would result in any benefits on downstream groundwater fed spring flows. They also say the pMEP restrictions are currently 100 % reliable but due to the declining trend in Wairau Aquifer levels restrictions are likely to become permanent at some point in the future, which is why MDC is focusing on what they call “alternative approaches to managing seasonal and boundary effects.” We are not clear what that actually means so will need to do some more research to learn more. It seems that any actions arising from the GBR research, to try and reduce or stop the declining trend in the aquifer are likely to be very expensive and to take decades to prove their worth. Suffice it to say it seems clear that this issue will be ongoing and not easy to resolve. The pMEP restriction regime is currently subject to appeals which should be heard some time in 2023.
Maia Hart has also done a good article on the GBR report in Stuff where she says - "New research suggests historic work to narrow the Wairau River could be contributing to declining levels in the recharge aquifer – one of Marlborough’s main water sources. The Wairau aquifer is the main groundwater system underlying the Wairau Plain and a source of irrigation, drinking and stock water. Water seeping from the Wairau River into the aquifer is the main ways it is recharged. Its levels have dropped since 1973, at rates unable to be explained by irrigation."