Water usage and efficiencies for irrigation in Northern Peru. A case study in Cajamarca, a region affected by mining industry

Froukje Kuijk, Prof. Guido Wyseure
Water use, Peru, Cajamarca

In Northern Peru water scarcity in the dry season is a real challenge, especially for agriculture. This issue augmented in the early 1990s by the introduction of Minera Yanacocha S.R.L, the second largest gold mine in the world. Both land use types address the same limiting water sources, inevitably leading to conflicts. Agriculture in this region mainly consists of pasture for livestock and dairy production, since it is robust to water scarcity. Water, distributed within irrigation commissions, is supplied by springs, in the higher Sierra, or via offtakes from rivers, closer to the valleys. Mainly border irrigation is performed, and within one irrigation network, water is conducted through open channels. In literature, evaluations of irrigation infrastructure, efficiency and distribution methods in the Northern Andes of Peru are scarce. The usage of
control structures to evaluate exact water quantities entering the channels, is barely implemented in this region.

The Yanacocha mine is positioned on the source area of several important rivers. Water pollution, disturbance of the hydrological distribution and reduction of the phreatic level are the main possible impacts of a mine on the fresh water environment. In this dissertation the focus is laid on the río Grande de Mashcón and its contributors. A discharge inventory in this catchment is carried out, based on historical and present measuring events, and trends of change in hydrology in time are derived. In the irrigation commission of
Tres Molinos three aspects are evaluated. Firstly the conveyance efficiency of the channel network is determined. The biomass production and irrigation productivity are simulated for different water supply scenarios. Finally the recently installed control structure is calibrated, making it ready to use.

It is observed that in the overall hydrology on catchment level a strong flow reduction occurred in the first years of the exploitation phase of the mine. Since the implementation of MYSRL’s more conscious water management, discharges in the rivers are stabilized, but the water withdraw from the open pits did continue. There is evidence for a significant groundwater reduction, affecting many, non-inventoried, springs in the higher Sierra.

The actual water quantity entering the channels of Tres Molinos does not reach the water license of 100lps in the dry season. The field water balance simulation demonstrated a biomassreduction of 15-17%. The performance within the irrigation network strongly depends upon the soil type and the conveyance efficiency. A water loss along the channel of 5.7lps.km-1 is observed to exist. Leakage through gates, robbery and removal of water for other purposes are identified as the main causes. In certain laterals improvement in channel infrastructure, distribution scheduling and evaluation of the suitability of distribution frequency relative to the soil characteristics is strongly recommended.

In Peru, decentralization of the power concerning water resources management is still in process. More support is still necessary for water allocation, infrastructure and efficiencies in irrigation commissions and in general in defending agricultural values in the highlands of Peru. The program of PSI Sierra is already a good incentive.