Watershed Information System

An online tool to efficiently organise, store and provide access to watershed information for collective water action

Water Balance

Maps Data and graphs

Key messages


Basic Information

Time frame water year 2017 (October 2016 – September 2017)

Type of year:

2017 is considered a wet year when looking at all precipitation records between 2012 and 2017 from the Agrimet stations within the Lower Boise (Average 2012-2017: 11.3 inches; average in 2017: 15.05 inches).
Units of the balance:  Acre-feet/year


Catchment Water Balance





Runoff at the outlet where the Lower Boise meets the Snake river at Parma


Snow water


Outflow to the Snake River and other subsurface discharges


Inflow from the Lucky Peak reservoir




Inflows from the Payette river and the Snake river (inter-basin water transfers)



Total Inflows


Total Outflows


Change in Storage = Inflows – Outflows = 769,211

Key messages:

  • The water balance presented for the Lower Boise watershed is based on public data.
  • In the water year 2017, considered a wet year, the water balance for the Lower Boise is positive, i.e. the change in storage is positive.
  • The values of various water balance components (Snow water and in and out inter-basin water transfers) have substantial levels of uncertainty as the water system in the Lower Boise is highly complex and data on the various components is not always easily accessible, not easily validated, or shows gaps.
  • 66% of all water consumed in the Lower Boise is consumed by agriculture: 46% by irrigated agriculture and 20% by non-irrigated agriculture.
  • 95% of the total water consumed by Agriculture in the Lower Boise is consumed by 10 crops, the category Herbs/Mint ranks 9th with a consumption of respectively 3% of total irrigated agricultural water consumption and 2% of total water consumption (irrigated and non-irrigated agriculture).
  • The consumption of water by the DCMI sectors (domestic, commercial, municipal and industrial) is estimated to be 6% of the total water consumption; water withdrawals by the DCMI sector in 2015 were 110,000 acre-feet/yr.
  • Consumptive water demand is expected to increase substantially in the next 50 years across all sectors.
  • Although there is uncertainty in the water balance, the assessment of the monthly ratio between total water consumption and total water available points to the months July and August 2017 as being critical months where total consumption is 66 and 73% of the total water available respectively.
  • Another major water issue is found in the Lower Boise between November and January: Environmental Flow Requirements are not complied at the inlet Actually, both issues are closely interconnected. The water is retained in the reservoirs upstream during late fall and winter, so that it can be released during the growing season for irrigation.
  • Climate change scenarios and expected trends in water availability (increase in yearly precipitation, change in peaks with less precipitation available in summer; and increase in summer evapotranspiration) indicate an increased challenge in managing water in reservoirs to comply with goals on flood control and irrigation demands.


Data and graphs

2 - Monthly ratio between total water consumption and total water available

Monthly Water balance for the Lower Boise Watershed during the 2017 water year in acre-feet/month.

Total water available = Precipitation + Snow + Rin

Consumption = ET

3 - Environmental Flow Requirements are not complied at the inlet

Lower Boise average monthly flow (cubic feet per second) below diversion dam from Lucky Peak (BDDI Hydromet station) and minimum in-stream flow recommendations (BREN, 2018). Minimum and preferred in-stream flow recommendations for the Lower Boise River (average cubic feet per second for each month of the year) * Both of these datasets appear in the literature review as the latest, however, we do not have a date of this literature review (Data found at BREN, 2018).


4 - Expected trends in water availability

An annual increase in precipitation of 1.5% is projected for the period 2021-2050, 3% for 2041-2070 and 5% for the period 2070-2099, compared to the period 1970 - 1999 (SPF Water, 2016). However, summer precipitation is projected to decrease throughout the Northwest by as much as 30% by the end of the century (idem).

The reservoir operations for and in the Lower Boise River Basin are designed based on historical timing of snowmelt runoff. With the increase of precipitation and a more erratic pattern in winter, dam operators will face more challenges in reconciling the competing goals of reservoir water storage and flood control releases. A shift in timing of peak flows could result in reservoirs having lower levels at the onset of spring, thereby creating water supply constraints (idem).