National systems are irrigation systems operated and maintained by NIA and constructed either with foreign loan assistance or purely locally funded. Foreign loan assisted projects are those whose funding requirements are provided for partly by foreign lending institutions, such as:
- Asian Development Bank (ADB)
- International Bank for Reconstruction & Development (IBRD)
- United States Agency for International Development (USAID)
- World Bank, and Overseas Economic Cooperation Fund (OECF) of Japan.
NIS usually has a service area of more than 1,000 hectares (has.). NIA jointly operates and maintains NIS with Irrigators Associations (IAs). Farmer-beneficiaries are charged with Irrigation Service Fee (ISF) for service rendered in the delivery of water. The agency operated and maintained 191 National Irrigation Systems (NISs) covering a total service area of 689,113 has. and benefiting 518,485 farmers nationwide.
In Region 5 (Bicol Region), there are eight (8) NIS covering a total service area of 20,487 has. These NIS are:
- Daet-Talisay River Irrigation System (DTRIS) and Matogdon RIS (MRIS) in Daet, Camarines Norte;
- Tigman-Hinagyanan-Inarihan River Irrigation System (THIRIS) in Magarao, Camarines Sur;
- Cagaycay River Irrigation System (CRIS) in San Jose, Camarines Sur;
- Rinconada Integrated Irrigation System (RIIS) in Sta. Elena, Iriga City;
- Libmanan-Cabusao Pump Irrigation System (LCPIS) in Libmanan, Camarines Sur;
- Mahaba-Nasisi-Ogsong-Hibiga River Irrigation System (MNOH RIS) in Tuburan, Ligao City; and
- San Ramon-San Francisco River Irrigation System (SRSFRIS) in Bulan, Sorsogon.