Viviana and I planned this project for months, but as often
happens with international development projects, nothing went according to
plan! After rearranging our schedule a couple times so the trip would take
place at the perfect point in my pregnancy (month 5!), Zika virus swept through
the Caribbean and Dominican Republic was placed on the CDC no-fly list. The
recommendation across the board at the time was that I shouldn’t travel to any
country with Zika, and so Viviana had to take the lead on the field design part
of the project with me assisting from back in California.
Besides
being disappointed to miss the trip (and the chance to meet with other UC Davis
International Agricultural Development friends working in Haiti!), this
situation introduced a lot of challenges. My job as the irrigation engineer was
to make sure all the pumps and pipes and sprinklers were going to be up to spec
and working, but now Viviana was going to be the one who would have to buy and
install all the equipment. We did our best to design as much as we could before
leaving, but it’s almost impossible to design things without getting the full
on the ground picture. Sprinkler systems are particularly challenging, because
the pump and the sprinklers depend on each other – the sprinklers require a
certain pressure and flow to operate, and if the pump gives them too much or
too little the system won’t work well. Without knowing exactly which sprinklers
would be available it was almost impossible to spec the system before hand.
Once she arrived, Viviana spent a
lot of time driving around to different supply stores and sites and getting all
the information that we needed. One of the biggest challenges in design is
getting everyone on the same page, and this project was no different. There are
a lot of design tradeoffs that have to be considered with a sprinkler irrigated
nursery. An ideal sprinkler pattern has a huge amount of overlap between
sprinklers. This ensures that the irregularity of the sprinkler pattern is
averaged out and the water will be uniformly applied. This sounds like a simple
idea, but it can be challenging to apply in practice. One of the biggest
questions we had to consider was what size sprinklers the nursery would use –
micro-sprinklers or standard sprinklers? Micro-sprinklers would have had a more
controlled application area, that would have avoided the pathways and edges of
the nursery, but we would have needed hundreds of them to cover the entire area
– a major undertaking for construction and maintenance. They also would have
had to be close to the ground, while the nursery operators preferred sprinklers
mounted high above the work area.
In order
to accommodate these preferences, we decide to use standard sprinklers with the
smallest diameter we could find. But deciding where to put them created more
tradeoffs – the local sprinkler distributor only had sprinklers that operated
in a full circle and none that operated in a half-circle. This means that the
nice straight edge we see in the diagram isn’t possible – either the edges of
the rows will have less uniform water distribution, or the sprinklers will have
to have half their radius hang out over the edge. In terms of uniformity, it
would be best to have the sprinklers line the edge of the nursery and irrigate
half the plants and half the pathways. But this isn’t ideal for a lot of other
reasons – it wastes water, gets the paths around the nursery all muddy, and
doesn’t feel like a well-designed system to the nursery workers. There are also
additional restrictions because of flow rates – if too much water comes out of
the sprinklers at once it will run off, which is a waste of both water and
power. So if there are limited sprinkler models available, there is also a
trade off between placing them close together and making sure that the flow is
not too high for the small pots to absorb. It was difficult to have to think
about all these design trade-offs and communicate about choices and
consequences via Viviana. She did am amazing job! But, it’s not the same as
being there in person and being able to have a back and forth conversation
between the engineer and the customer.
Ultimately,
I would say the design was about 85% optimized. I wish that it was 100%, but
85% is pretty incredible all things considered! The nursery bought a great pump
– it is flexible enough to run off grid power or a solar panel if they wish, it
can pump water from the river into the cistern or from the cistern to the
nursery, and it provides pretty much the exact amount of pressure and flow
required. Success! Viviana was able to work with the pump technician to get
pressure regulating valves downstream of the pump so that the pressure to the
sprinklers would be appropriate and consistent – a very important addition if
they wanted the flexibility to either use the pump from the river down the hill
or from the local supply at the same level. The installed system used PVC
delivery pipes and risers, which are not always necessary, but were what the
customer preferred given concerns about rodent damage. And the sprinklers
matched the needs of the system in terms of flow rate and spacing. All in all,
I was super happy with how the project turned out – there are so many
challenges that come up in international projects, especially small-scale
projects like this where locally sourced materials are a key consideration.
Viviana did an amazing job managing all the equipment and expectations on the
ground, and getting everything ready for – the big reveal!
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