Rainwater harvesting is the one of the main components of our long term strategy to build in situ (localized) farm irrigation systems. The development of in situ (localized) farm irrigation systems is vital because most of the agricultural lands in the Philippines are mainly rolling hills or in upland areas; thus are not considered as areas suitable for conventional irrigation systems. Further, after more than fifty years of existence, the National Irrigation Agency (NIA) has reported only 1.67 million hectares as irrigated out of the total 3.1 million irrigable lands (with elevations not exceeding 3o).
About 8 million hectares of the agricultural lands in the Philippines remain unirrigated; thus, most are cultivated mainly when there is sufficient rainwater during the rainy season. Drought-tolerant crops may be cultivated during the dry season but the risk of prolonged scarcity of rainfall may lead to severe drought that would either reduce the crop yield or even lead tocomplete crop failure.
Storage and percolation farm ponds are key components of our in situ (localized) farm irrigation system planned projects. In the Reference Desk section, we re-printed segments of the manual of Farm pond construction, management and maintenance published by the US Army Corps of Engineers, as well as similar internet documents from other reliable sources, to serve as guidelines for proper construction of storage and percolation farm ponds.
We included videos revealing inspiring efforts in even the most arid places in the world, where rainfall could be as low as 300-400 mm annually and yet through collaborative and dedicated work from the farmers in the community, these desert lands have been converted to green oasis even in the backdrop of drought, climate change and global warming.
Thus, in wet tropical countries, like the Philippines, where the lowest mean annual rainfall (I have encountered so far) was around 670mm. Most regions of the Philippines would have annual rainfall exceeding 1000mm while other wet regions would exceed 2000mm or higher annually. As such, the Philippines is considered to have "sufficient", if not an over-abundance of rainwater.
And yet, the Philippines experience water scarcity or even drought regularly during the dry season and especially during the occurrence of El Niño.
One of the long term goals of Kalikasan-Philippines is to address the question: How can we harvest ALL the excess rainwater during the rainy season, so that the stored water will be available during scarcity of rain during the dry season to be used for domestic consumption, crop cultivation, animal husbandry and aquaculture?
Groundwater recharge. If constructed properly, farm ponds would serve as rainwater storage. Also, large and deep ponds would be one of the strategies we plan to integrate to store, if possible, ALL the rainwater that would collect in a given region; and, the ponds would serve as one of the mechanisms used for groundwater recharge.
Aside from surface water storage in deep ponds and the connecting canals, groundwater recharge is is key in the grand design of the in situ (localized) farm irrigation system projects that we envision. There will be years during a long and strong La Niña occurrence so that there would be so much excess rainwater that needed to be stored through groundwater recharge so that there would be a water source that could be tapped naturally during periods of long and strong El Niño occurrence, usually accompanied by less rain, and hight temperature that lead to drought.
However, even without the severe contrasting influence of El Niño and La Niña, the long rainy season bring more rainwater that should be stored in farm ponds and through groundwater recharge so that they will be available for use during the dry season that occur in this particular area from January to May. While short and heavy rains start to arrive in May then more in June, the exposed ground (prevalent in monoculture agriculture practiced in the Philippines) are parched dry and absorbs most of the rainwater that is not immediately lost to evaporation and flash flood drainage systems.
Farm Ponds Constructions. It is heartening to observe that building of farm ponds have been started in some areas visited in 2014. However, as shown in this farm pond, many have not been constructed properly so that their use as rainwater storage areas are ineffective.
Pond Depth to Slope Ratio. Instead of the 1:3 or at the very least 1:2 ratio of height to inclined slope of the pond banks, the pond shown in the images has very steep banks, thus very prone to erosion and subsequent siltation of the pond. However, these ratios would significantly reduce the storage capacity of the ponds, especially smaller ponds. The impact of depth to slope ration on water storage capacity diminishes where larger and deeper ponds can be constructed. Thus, while rainwater harvesting is feasible for any area of land, it is more effective if the in situ (localized) farm irrigation system projects that we envision will comprise of larger areas of contiguous farms and human habitation.
To reduce the surface area of the ponds to minimize evaporation losses, we will explore depths of six (6) meters and more. Also, this will store more water per unit area to minimize conversion of farm lands, to ponds in case there is no adjacent wetland or flood plain practical for surface rainwater storage. However, this depths may lead to complications, like low oxygen levels at deeper areas. This could lead to anaerobic conditions that triggers the production of greenhouses gases, like methane. Also, the lack of sufficient oxygen in the deeper water levels could impact fish growth, if the storage ponds serve multi-purgpose uses, e.g., aquaculture or aquaponics.
To address the issue, one of our planned projects will explore the use of sustainable energy resources, readily available in the area (biomass, solar and wind resources) to drive the pumps and machineries to promote aeration in the deep ponds. Once the projects are underway, in consultation with SEAFDEC and other water agencies and research institution in the Philippines and potential international collaborators, we will explore the use of freshwater aquatic plants to aid in dissolve oxygen balance stabilization in deepwater ponds.
Bank Protection. Flash flooding that is common in compact soil, especially in rolling terrain areas brings with it, suspended soil, other debris and unwanted organisms. More natural bank protection, e.g., use of vetiver and riparian plants, may be planted around the periphery of the bank. These will serve to strengthen and anchor the bank to mitigate bank erosion, as well as serve as filtration system to prevent silt and debris from entering the farm pond.
Windbreaks and Farm Forest Zones. The group of trees, shrubs and plants that will be used to create the windbreak zones may be planned to create local and miniature rainforest zones in strategic locations of the farm. A well selected biodiverse mix of tall trees, nitrogen-fixing plants (and trees), fruit-bearing trees, shrubs and plants, shade loving understory trees and plants, shade loving soil surface cover, and more may be created. This will attract birds, bees, insects and other organisms, and with the addition of surface water ecosystems (e.g., ponds, swales, irrigation canals) attract aquatic, amphibians and other water loving organisms
Multi-purpose uses of the farm ponds. If constructed properly, it is most advantageous to explore the use of the ponds for other uses, especially aquaculture or better still a large scale version of aquaponics. The addition of aquatic organisms, including fish, is critical for a number of reasons. Standing water would serve as natural habitat for many organisms, including desirable and undesirable organisms. The ponds would be breeding grounds for mosquitoes and other unwanted organism. The addition of fish and attraction of other aquatic, amphibians or water loving organisms would serve to control the population of unwanted organisms.
Biological Pest Control. The addition of water ecosystems increases biodiversity and ecodiversity of the resulting ecosystem. This will increase the stability and provide more sustainable checks and balances to control the population mix of the flora and fauna. Thus, biological pest control may be achievable because of the resulting biodiversity and without the use of industrial pesticides and biocides that would add to the cost of farm operation but also deleterious to human health and de-stabilize the ecosystem.
From more practical and economic perspectives the biodiversity and ecodiversity of the resulting EcoCulture farm would diversify the potential sources of income of the farmer, shielding the family from unwanted surprises common in farming, i.e., lower crop yield or complete crop failure due to flash floods, drought, or pest infestation.